diff --git a/application/editormode.h b/application/editormode.h
index 987ba5d3..d55ffc09 100644
--- a/application/editormode.h
+++ b/application/editormode.h
@@ -39,6 +39,7 @@ http://mozilla.org/MPL/2.0/.
 #include "editor/editorSpeed.hpp"
 #include "editor/editorRoad.hpp"
 #include "editor/editorProfile.hpp"
+#include "editor/editorGroundFollow.hpp"
 #include "editor/editorInfra.hpp"
 #include "editor/editorGauge.hpp"
 #include "editor/editorClips.hpp"
@@ -1512,6 +1513,8 @@ class editor_mode : public application_mode, private editor_track::observer, pri
 	bool start_extend(editor_track::snap_target const &End);
 	void finish_extend();
 	std::vector<segment_data> extend_pieces() const;
+	// the same for the track shown while it's dragged: its grade line fitted on the thread of the fitter, the frame not waiting for it
+	std::vector<segment_data> extend_preview() const;
 	// the free end of the path as the start of a new track; false if it isn't free or has no direction
 	static bool free_end(TTrack &Track, int const Path, bool const Atend, extend_tool &Tool);
 	// the free end new pieces are added at: the selected point if it's one, else the end, else the start
@@ -1732,6 +1735,10 @@ class editor_mode : public application_mode, private editor_track::observer, pri
 	static std::pair<double, double> trackbed(TTrack const *Style);
 	std::vector<double> bed_heights(std::vector<glm::dvec3> const &Points, std::vector<glm::dvec2> const &Directions, TTrack const *Style) const;
 	void follow_ground(std::vector<segment_data> &Pieces, double const Height0, double const Grade0, double const Height1, double const Grade1, TTrack const *Style) const;
+	ground_follow::grading laying_grading(TTrack const *Style) const;
+	// the grade lines of the tracks shown while they're laid are fitted on a thread of their own, not to hold up the frames
+	mutable ground_follow::fitter m_fitter;
+	mutable bool m_follow_preview{false};
 	glm::dvec3 lay_bed(glm::dvec3 const &Point);
 	std::optional<glm::dvec3> m_cursor_override;
 	bool sweep_grabs(glm::dvec3 const &Point);
diff --git a/application/editormodelay.cpp b/application/editormodelay.cpp
index 9e4e6bcb..53e3a7fa 100644
--- a/application/editormodelay.cpp
+++ b/application/editormodelay.cpp
@@ -101,7 +101,9 @@ void editor_mode::update_build_tools()
 		if (plan_distance(lay.mouse, points.back()) > 1.0)
 			points.push_back(lay.mouse);
 	}
+	m_follow_preview = true;
 	lay.preview = lay_pieces(points, end, lay.preview_length, lay.preview_error);
+	m_follow_preview = false;
 }
 
 bool editor_mode::free_end(TTrack &Track, int const Path, bool const Atend, extend_tool &Tool)
@@ -490,6 +492,14 @@ std::vector<segment_data> editor_mode::extend_pieces() const
 	return pieces;
 }
 
+std::vector<segment_data> editor_mode::extend_preview() const
+{
+	m_follow_preview = true;
+	auto pieces{extend_pieces()};
+	m_follow_preview = false;
+	return pieces;
+}
+
 TTrack const *editor_mode::speed_track(TTrack const *From, TTrack const *To) const
 {
 	if (m_lay.own_speed && m_lay.style.velocity > 0.0)
diff --git a/application/editormodenmt.cpp b/application/editormodenmt.cpp
index e13e5c2e..9630cb94 100644
--- a/application/editormodenmt.cpp
+++ b/application/editormodenmt.cpp
@@ -691,6 +691,10 @@ void editor_mode::nmt_follow_tracks()
 		return;
 	if (state.stage != nmt_state::automatic::idle || state.fitting || ImGui::GetTime() - state.changed_time < 0.3 || ImGui::IsMouseDown(ImGuiMouseButton_Left) || ImGui::IsMouseDown(ImGuiMouseButton_Right))
 		return;
+	// laying on, from the end of the track laid or dragged from a free end, the terrain waits for the laying to end: shaping it
+	// goes through every chunk under the tracks, a stall each click
+	if ((m_lay.active && false == m_lay.points.empty()) || m_extend.active)
+		return;
 	if (false == m_streamer.active())
 	{
 		state.changed.clear();
diff --git a/application/editormodeselftest.cpp b/application/editormodeselftest.cpp
index 749ad391..b74389cb 100644
--- a/application/editormodeselftest.cpp
+++ b/application/editormodeselftest.cpp
@@ -151,7 +151,11 @@ editor_mode::selftest_result editor_mode::selftest_dispatch(std::string const &c
 //   screenshot
 //   save
 //   log <text>
-//   perf <label>                         the frames drawn since the last perf and the time per frame logged, with the timings and counts of the renderer
+//   perf <label>                         the frames drawn since the last perf and the time per frame logged, the longest frame and those over 33 ms,
+//                                        with the timings and counts of the renderer
+//   fittime <metres>...                  the grade line of a straight over rolling ground of each length fitted, as at a click, and the time the
+//                                        frame waits for it while the track is laid, the line fitted on the thread of the fitter
+//   terrainhash                          a hash of the heights of the terrain chunks in memory, to compare the ground two builds made
 //   rebuildall                           the geometry of every path in the sections made already laid anew, the time it took logged,
 //                                        and the 3d sleepers of all the tracks laid anew, timed apart
 //   sleepercheck                         the 3d sleepers of all the tracks checked for beams over each other and rails without enough under them
@@ -177,6 +181,16 @@ void editor_mode::selftest_step()
 		set_norms_free(true);
 	}
 	++editor_selftest::frames();
+	// how long each frame took, for the longest one perf tells
+	{
+		static auto last{std::chrono::steady_clock::now()};
+		auto const now{std::chrono::steady_clock::now()};
+		auto const frame{std::chrono::duration<double, std::milli>(now - last).count()};
+		last = now;
+		editor_selftest::frame_longest() = std::max(editor_selftest::frame_longest(), frame);
+		if (frame > 33.0)
+			++editor_selftest::frames_slow();
+	}
 	if (test.wait > 0)
 	{
 		--test.wait;
diff --git a/application/editormodeselftest.h b/application/editormodeselftest.h
index cd3c70be..e9468f66 100644
--- a/application/editormodeselftest.h
+++ b/application/editormodeselftest.h
@@ -24,6 +24,19 @@ inline std::size_t &frames()
 	return count;
 }
 
+// the longest frame since the last perf, and the frames longer than 33 ms
+inline double &frame_longest()
+{
+	static double value{0.0};
+	return value;
+}
+
+inline std::size_t &frames_slow()
+{
+	static std::size_t count{0};
+	return count;
+}
+
 inline std::string one_line(std::string Text)
 {
 	std::replace(Text.begin(), Text.end(), '\n', ';');
diff --git a/application/editormodeselftestjoin.cpp b/application/editormodeselftestjoin.cpp
index 93b79898..652f62a2 100644
--- a/application/editormodeselftestjoin.cpp
+++ b/application/editormodeselftestjoin.cpp
@@ -129,6 +129,35 @@ editor_mode::selftest_result editor_mode::selftest_join(std::string const &comma
 			WriteLog(format("SELFTEST %s end: %.3f m from the target", command.c_str(), geometry::plan_distance(previous, b)));
 		return selftest_result::handled;
 	}
+	if (command == "fittime")
+	{
+		// the grade line of a straight laid over rolling ground fitted, as at a click, and as while the track follows the
+		// cursor: the frame then waits only for the ground to be sampled
+		for (double length{0.0}; words >> length;)
+		{
+			auto const ground = [](double const X) { return 130.0 + 4.0 * std::sin(X / 180.0) + 1.5 * std::sin(X / 37.0) + 0.2 * std::sin(X / 6.0); };
+			auto const bed = [&](std::vector<glm::dvec3> const &Points, std::vector<glm::dvec2> const &) {
+				std::vector<double> heights;
+				for (auto const &point : Points)
+					heights.push_back(ground(point.x));
+				return heights;
+			};
+			auto const pieces{geometry::straight_piece({0.0, ground(0.0), 0.0}, {1.0, 0.0}, 0.0, length)};
+			ground_follow::ends const ends{ground(0.0), std::numeric_limits<double>::quiet_NaN(), ground(length), std::numeric_limits<double>::quiet_NaN()};
+			auto const grading{laying_grading(nullptr)};
+			auto fitted{pieces};
+			auto const start{std::chrono::steady_clock::now()};
+			ground_follow::follow(fitted, ends, bed, grading);
+			auto const fit{std::chrono::duration<double, std::milli>(std::chrono::steady_clock::now() - start).count()};
+			ground_follow::fitter fitter;
+			auto shown{pieces};
+			auto const asked{std::chrono::steady_clock::now()};
+			ground_follow::follow_meanwhile(shown, ends, bed, grading, fitter);
+			auto const meanwhile{std::chrono::duration<double, std::milli>(std::chrono::steady_clock::now() - asked).count()};
+			WriteLog(format("SELFTEST fittime %.0f m: fitted in %.1f ms, %zu pieces; while laid %.2f ms", length, fit, fitted.size(), meanwhile));
+		}
+		return selftest_result::handled;
+	}
 	if (command == "layheights")
 	{
 		std::string which;
diff --git a/application/editormodeselftestscene.cpp b/application/editormodeselftestscene.cpp
index 1102c175..f670abd1 100644
--- a/application/editormodeselftestscene.cpp
+++ b/application/editormodeselftestscene.cpp
@@ -113,9 +113,11 @@ editor_mode::selftest_result editor_mode::selftest_scene(std::string const &comm
 		auto const now{std::chrono::steady_clock::now()};
 		auto const frames{editor_selftest::frames() - lastframe};
 		auto const elapsed{std::chrono::duration<double, std::milli>(now - last).count()};
-		WriteLog(format("SELFTEST perf %s: %zu frames, %.3f ms per frame | %s | %s", label.c_str(), frames, frames > 0 ? elapsed / frames : 0.0, editor_selftest::one_line(GfxRenderer->info_times()).c_str(), editor_selftest::one_line(GfxRenderer->info_stats()).c_str()));
+		WriteLog(format("SELFTEST perf %s: %zu frames, %.3f ms per frame, the longest %.1f ms, %zu over 33 ms | %s | %s", label.c_str(), frames, frames > 0 ? elapsed / frames : 0.0, editor_selftest::frame_longest(), editor_selftest::frames_slow(), editor_selftest::one_line(GfxRenderer->info_times()).c_str(), editor_selftest::one_line(GfxRenderer->info_stats()).c_str()));
 		last = now;
 		lastframe = editor_selftest::frames();
+		editor_selftest::frame_longest() = 0.0;
+		editor_selftest::frames_slow() = 0;
 		return selftest_result::handled;
 	}
 	if (command == "rebuildall")
diff --git a/application/editormodeselftestterrain.cpp b/application/editormodeselftestterrain.cpp
index 7cdbf4b0..dc70072e 100644
--- a/application/editormodeselftestterrain.cpp
+++ b/application/editormodeselftestterrain.cpp
@@ -42,6 +42,7 @@ http://mozilla.org/MPL/2.0/.
 
 #include <chrono>
 #include <cstdint>
+#include <cstring>
 #include <cstdlib>
 #include <fstream>
 #include <map>
@@ -470,6 +471,23 @@ editor_mode::selftest_result editor_mode::selftest_terrain(std::string const &co
 		WriteLog("SELFTEST terrainadd: " + m_terrain_status);
 		return selftest_result::handled;
 	}
+	if (command == "terrainhash")
+	{
+		// the heights of the chunks in memory hashed, to compare the ground two builds made
+		std::vector<editor_terrain *> chunks;
+		m_streamer.collect(chunks);
+		std::sort(chunks.begin(), chunks.end(), [](editor_terrain const *A, editor_terrain const *B) { return A->key() < B->key(); });
+		std::uint64_t hash{1469598103934665603ull};
+		for (auto const *chunk : chunks)
+			for (auto const height : chunk->heights())
+			{
+				std::uint32_t bits;
+				std::memcpy(&bits, &height, sizeof(bits));
+				hash = (hash ^ bits) * 1099511628211ull;
+			}
+		WriteLog(format("SELFTEST terrainhash: %zu chunks, %016llx", chunks.size(), static_cast<unsigned long long>(hash)));
+		return selftest_result::handled;
+	}
 	if (command == "terrainprobe")
 	{
 		double x{0.0};
diff --git a/application/editormodetrack.cpp b/application/editormodetrack.cpp
index 8301bd94..8b800cd9 100644
--- a/application/editormodetrack.cpp
+++ b/application/editormodetrack.cpp
@@ -1832,7 +1832,7 @@ std::string editor_mode::track_readout() const
 	{
 		double length{0.0};
 		double radius{0.0};
-		auto const pieces{extend_pieces()};
+		auto const pieces{extend_preview()};
 		for (auto const &piece : pieces)
 		{
 			length += bezier{piece}.plan_length();
@@ -2238,7 +2238,7 @@ void editor_mode::draw_build_overlay() const
 			ImVec2 screen;
 			if (projection.project(target.position, screen))
 				drawlist->AddCircle(screen, 14.0f, IM_COL32(255, 60, 255, 255), 20, 3.0f);
-			for (auto const &piece : extend_pieces())
+			for (auto const &piece : extend_preview())
 				drawpath(piece, overlay_color::marked);
 		}
 		else if (plan_crossover(plan))
@@ -2249,7 +2249,7 @@ void editor_mode::draw_build_overlay() const
 		}
 		else
 		{
-			for (auto const &piece : extend_pieces())
+			for (auto const &piece : extend_preview())
 				drawpath(piece, overlay_color::marked);
 		}
 	}
diff --git a/application/editormodetrackbed.cpp b/application/editormodetrackbed.cpp
index a51dc07c..cae113d4 100644
--- a/application/editormodetrackbed.cpp
+++ b/application/editormodetrackbed.cpp
@@ -134,6 +134,16 @@ void editor_mode::follow_ground(std::vector<segment_data> &Pieces, double const
 		ground_follow::follow(Pieces, {Height0, Grade0, Height1, Grade1}, [](std::vector<glm::dvec3> const &Points, std::vector<glm::dvec2> const &) { return std::vector<double>(Points.size(), std::numeric_limits<double>::quiet_NaN()); });
 		return;
 	}
+	auto const grading{laying_grading(Style)};
+	auto const bed = [&](std::vector<glm::dvec3> const &Points, std::vector<glm::dvec2> const &Directions) { return bed_heights(Points, Directions, Style); };
+	if (m_follow_preview)
+		ground_follow::follow_meanwhile(Pieces, {Height0, Grade0, Height1, Grade1}, bed, grading, m_fitter);
+	else
+		ground_follow::follow(Pieces, {Height0, Grade0, Height1, Grade1}, bed, grading, &m_fitter);
+}
+
+ground_follow::grading editor_mode::laying_grading(TTrack const *Style) const
+{
 	// the line goes over the ground as the tracks go onto the terrain: the vertical curves of the norms, the grades as steep as the
 	// ground has them, a rise or a hollow longer than a structure followed closely, the bumps and what parts from it gone past
 	ground_follow::grading grading;
@@ -148,7 +158,7 @@ void editor_mode::follow_ground(std::vector<segment_data> &Pieces, double const
 	grading.fit.follow_steepest = 100.0;
 	grading.fit.follow_tolerance = 0.05;
 	grading.reach = kModel;
-	ground_follow::follow(Pieces, {Height0, Grade0, Height1, Grade1}, [&](std::vector<glm::dvec3> const &Points, std::vector<glm::dvec2> const &Directions) { return bed_heights(Points, Directions, Style); }, grading);
+	return grading;
 }
 
 glm::dvec3 editor_mode::lay_bed(glm::dvec3 const &Point)
diff --git a/editor/editorGroundFollow.cpp b/editor/editorGroundFollow.cpp
index d30030a8..926a82e5 100644
--- a/editor/editorGroundFollow.cpp
+++ b/editor/editorGroundFollow.cpp
@@ -62,13 +62,47 @@ samples sample(std::vector<segment_data> const &Pieces, std::vector<double> cons
 	return result;
 }
 
-bool grade_line(samples const &Points, std::vector<double> const &Targets, double const Total, ends const &Ends, grading const &Grading, profile::line &Line)
+bool same(double const A, double const B)
 {
+	return A == B || (std::isnan(A) && std::isnan(B));
+}
+
+bool same(std::vector<double> const &A, std::vector<double> const &B)
+{
+	return A.size() == B.size() && std::equal(A.begin(), A.end(), B.begin(), [](double const X, double const Y) { return same(X, Y); });
+}
+
+bool same(ends const &A, ends const &B)
+{
+	return same(A.height0, B.height0) && same(A.grade0, B.grade0) && same(A.height1, B.height1) && same(A.grade1, B.grade1);
+}
+
+bool same(grading const &A, grading const &B)
+{
+	auto const &a{A.settings};
+	auto const &b{B.settings};
+	auto const &an{a.norms};
+	auto const &bn{b.norms};
+	auto const &af{A.fit};
+	auto const &bf{B.fit};
+	// the laying tells no points, humps nor bounds: a grading with any isn't taken for another
+	return a.points.empty() && b.points.empty() && a.speed == b.speed && a.join_start == b.join_start && a.join_end == b.join_end && an.grade_max == bn.grade_max && an.grade_switch_max == bn.grade_switch_max &&
+	       an.radius_factor == bn.radius_factor && an.radius_min == bn.radius_min && an.curve_threshold == bn.curve_threshold && an.element_min == bn.element_min && an.fixed_tolerance == bn.fixed_tolerance &&
+	       af.tolerance == bf.tolerance && af.grade_max == bf.grade_max && af.follow_length == bf.follow_length && af.follow_height == bf.follow_height && af.follow_steepest == bf.follow_steepest &&
+	       af.follow_tolerance == bf.follow_tolerance && af.humps.empty() && bf.humps.empty() && af.hump_radius == bf.hump_radius && af.bounds.empty() && bf.bounds.empty() && af.hold_grades == bf.hold_grades &&
+	       A.reach == B.reach;
+}
+
+bool grade_line(request const &Asked, profile::line &Line)
+{
+	auto const Total{Asked.total};
+	auto const &Ends{Asked.heights};
+	auto const &Grading{Asked.grades};
 	auto const none{std::numeric_limits<double>::quiet_NaN()};
 	std::vector<double> chainages{0.0};
 	std::vector<double> ground{none};
-	chainages.insert(chainages.end(), Points.chainages.begin(), Points.chainages.end());
-	ground.insert(ground.end(), Targets.begin(), Targets.end());
+	chainages.insert(chainages.end(), Asked.chainages.begin(), Asked.chainages.end());
+	ground.insert(ground.end(), Asked.targets.begin(), Asked.targets.end());
 	chainages.push_back(Total);
 	ground.push_back(none);
 	if (std::none_of(ground.begin(), ground.end(), [](double const Height) { return std::isfinite(Height); }))
@@ -151,6 +185,61 @@ void lift(segment_data &Piece, std::pair<double, double> const &Span, std::funct
 	control2 = {-along.x, -end * length / 3.0, -along.y};
 }
 
+// the pieces cut where the grade of the line breaks and lifted onto it, the chainages of the line Scale times those of the
+// pieces; with no line the heights lead from one end to the other
+void lay_on(std::vector<segment_data> &Pieces, std::vector<double> const &Lengths, double const Total, ends const &Ends, profile::line const *Graded, double const Scale)
+{
+	std::vector<std::pair<double, double>> spans;
+	if (Graded != nullptr)
+	{
+		auto knots{profile::breaks(*Graded)};
+		for (auto &knot : knots)
+			knot /= Scale;
+		// the ends of the track go on in the height and the grade of the tracks they join, whatever the line found
+		auto result{cut(Pieces, Lengths, knots, spans)};
+		for (std::size_t i = 0; i < result.size(); ++i)
+			lift(result[i], spans[i], [&](double const Chainage, double &Grade) {
+				if (Chainage < 1e-6 && false == std::isnan(Ends.grade0))
+				{
+					Grade = Ends.grade0;
+					return Ends.height0;
+				}
+				if (Chainage > Total - 1e-6 && false == std::isnan(Ends.grade1))
+				{
+					Grade = Ends.grade1;
+					return Ends.height1;
+				}
+				Grade = profile::grade(*Graded, Chainage * Scale) * Scale;
+				return profile::elevation(*Graded, Chainage * Scale);
+			});
+		Pieces = std::move(result);
+		return;
+	}
+	knot_line line{{0.0, Total}, {Ends.height0, Ends.height1}, {}, Ends.grade0, Ends.grade1};
+	line.refresh();
+	auto result{cut(Pieces, Lengths, line.knots, spans)};
+	for (std::size_t i = 0; i < result.size(); ++i)
+		lift(result[i], spans[i], [&](double const Chainage, double &Grade) { return line.at(Chainage, Grade); });
+	Pieces = std::move(result);
+}
+
+// the lengths of the pieces in plan and the ground sampled along them; returns the length of them all
+double measure(std::vector<segment_data> const &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading, std::vector<double> &Lengths, request &Asked)
+{
+	double total{0.0};
+	for (auto const &piece : Pieces)
+		total += Lengths.emplace_back(bezier{piece}.plan_length(256));
+	if (total < 1e-3)
+		return total;
+	auto points{sample(Pieces, Lengths, total, Ends)};
+	Asked.targets = Bed(points.points, points.directions);
+	Asked.chainages = std::move(points.chainages);
+	Asked.total = total;
+	Asked.heights = Ends;
+	Asked.grades = Grading;
+	return total;
+}
+
 } // namespace
 
 void knot_line::refresh()
@@ -177,45 +266,109 @@ double knot_line::at(double const Chainage, double &Grade) const
 	return geometry::hermite(length, heights[i], slopes[i], heights[i + 1], slopes[i + 1], std::clamp((Chainage - knots[i]) / length, 0.0, 1.0), Grade);
 }
 
-void follow(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading)
+bool request::operator==(request const &Other) const
+{
+	return total == Other.total && same(heights, Other.heights) && chainages == Other.chainages && same(targets, Other.targets) && same(grades, Other.grades);
+}
+
+fitter::~fitter()
+{
+	{
+		std::lock_guard<std::mutex> lock{m_mutex};
+		m_stop = true;
+	}
+	m_wake.notify_all();
+	if (m_thread.joinable())
+		m_thread.join();
+}
+
+bool fitter::done(request const &Request, profile::line &Line, bool &Graded) const
+{
+	std::lock_guard<std::mutex> lock{m_mutex};
+	if (false == m_result.has_value() || false == (m_result->asked == Request))
+		return false;
+	Line = m_result->line;
+	Graded = m_result->graded;
+	return true;
+}
+
+bool fitter::latest(request &Request, profile::line &Line, bool &Graded) const
+{
+	std::lock_guard<std::mutex> lock{m_mutex};
+	if (false == m_result.has_value())
+		return false;
+	Request = m_result->asked;
+	Line = m_result->line;
+	Graded = m_result->graded;
+	return true;
+}
+
+void fitter::ask(request Request)
+{
+	{
+		std::lock_guard<std::mutex> lock{m_mutex};
+		if ((m_asked.has_value() && *m_asked == Request) || (m_fitting.has_value() && *m_fitting == Request) || (m_result.has_value() && m_result->asked == Request))
+			return;
+		m_asked = std::move(Request);
+		if (false == m_thread.joinable())
+			m_thread = std::thread{[this]() { work(); }};
+	}
+	m_wake.notify_one();
+}
+
+void fitter::work()
+{
+	std::unique_lock<std::mutex> lock{m_mutex};
+	for (;;)
+	{
+		m_wake.wait(lock, [this]() { return m_stop || m_asked.has_value(); });
+		if (m_stop)
+			return;
+		m_fitting = std::move(m_asked);
+		m_asked.reset();
+		result fitted{*m_fitting};
+		lock.unlock();
+		fitted.graded = grade_line(fitted.asked, fitted.line);
+		lock.lock();
+		m_result = std::move(fitted);
+		m_fitting.reset();
+	}
+}
+
+void follow(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading, fitter const *Fitted)
 {
 	std::vector<double> lengths;
-	double total{0.0};
-	for (auto const &piece : Pieces)
-		total += lengths.emplace_back(bezier{piece}.plan_length(256));
+	request asked;
+	auto const total{measure(Pieces, Ends, Bed, Grading, lengths, asked)};
 	if (total < 1e-3)
 		return;
-	auto const points{sample(Pieces, lengths, total, Ends)};
-	auto const targets{Bed(points.points, points.directions)};
-	std::vector<std::pair<double, double>> spans;
-	if (profile::line graded; grade_line(points, targets, total, Ends, Grading, graded))
+	profile::line graded;
+	bool found{false};
+	if (Fitted == nullptr || false == Fitted->done(asked, graded, found))
+		found = grade_line(asked, graded);
+	lay_on(Pieces, lengths, total, Ends, found ? &graded : nullptr, 1.0);
+}
+
+void follow_meanwhile(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading, fitter &Fitter)
+{
+	std::vector<double> lengths;
+	request asked;
+	auto const total{measure(Pieces, Ends, Bed, Grading, lengths, asked)};
+	if (total < 1e-3)
+		return;
+	profile::line graded;
+	bool found{false};
+	if (Fitter.done(asked, graded, found))
 	{
-		// the ends of the track go on in the height and the grade of the tracks they join, whatever the line found
-		auto result{cut(Pieces, lengths, profile::breaks(graded), spans)};
-		for (std::size_t i = 0; i < result.size(); ++i)
-			lift(result[i], spans[i], [&](double const Chainage, double &Grade) {
-				if (Chainage < 1e-6 && false == std::isnan(Ends.grade0))
-				{
-					Grade = Ends.grade0;
-					return Ends.height0;
-				}
-				if (Chainage > total - 1e-6 && false == std::isnan(Ends.grade1))
-				{
-					Grade = Ends.grade1;
-					return Ends.height1;
-				}
-				Grade = profile::grade(graded, Chainage);
-				return profile::elevation(graded, Chainage);
-			});
-		Pieces = std::move(result);
+		lay_on(Pieces, lengths, total, Ends, found ? &graded : nullptr, 1.0);
 		return;
 	}
-	knot_line line{{0.0, total}, {Ends.height0, Ends.height1}, {}, Ends.grade0, Ends.grade1};
-	line.refresh();
-	auto result{cut(Pieces, lengths, line.knots, spans)};
-	for (std::size_t i = 0; i < result.size(); ++i)
-		lift(result[i], spans[i], [&](double const Chainage, double &Grade) { return line.at(Chainage, Grade); });
-	Pieces = std::move(result);
+	// the cursor moves on a little from frame to frame: the line fitted for where it was a moment ago is near enough, as long
+	// as the track goes on from the same end
+	request last;
+	auto const stretched{Fitter.latest(last, graded, found) && found && last.total > 1e-3 && same(last.heights.height0, Ends.height0) && same(last.heights.grade0, Ends.grade0)};
+	Fitter.ask(std::move(asked));
+	lay_on(Pieces, lengths, total, Ends, stretched ? &graded : nullptr, stretched ? last.total / total : 1.0);
 }
 
 } // namespace ground_follow
diff --git a/editor/editorGroundFollow.hpp b/editor/editorGroundFollow.hpp
index cb843996..aa45216f 100644
--- a/editor/editorGroundFollow.hpp
+++ b/editor/editorGroundFollow.hpp
@@ -9,7 +9,11 @@ http://mozilla.org/MPL/2.0/.
 
 #pragma once
 
+#include <condition_variable>
 #include <functional>
+#include <mutex>
+#include <optional>
+#include <thread>
 #include <vector>
 #include <glm/glm.hpp>
 
@@ -49,7 +53,54 @@ struct grading
 	double reach{100.0}; // m, how far the ground may part from the line and still be followed
 };
 
-// where Bed finds no ground, the heights lead from one end to the other
-void follow(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading = {});
+// the ground under a track, sampled along it, and what the grade line is fitted to
+struct request
+{
+	std::vector<double> chainages;
+	std::vector<double> targets; // the height of the bed at each chainage, NaN where there's no ground
+	double total{0.0}; // m, the length in plan
+	ends heights;
+	grading grades;
+
+	bool operator==(request const &Other) const;
+};
+
+// the grade lines of the track shown while it's laid, fitted on a thread of their own: fitting one takes some milliseconds,
+// tens of them on a long track over the terrain model, and the track follows the cursor frame after frame. The latest one
+// asked for is fitted, the ones asked for meanwhile are left out
+class fitter
+{
+  public:
+	~fitter();
+	// the line of exactly this request, once fitted; graded false: none was found, the heights lead from end to end
+	bool done(request const &Request, profile::line &Line, bool &Graded) const;
+	// the line fitted last, of whatever request
+	bool latest(request &Request, profile::line &Line, bool &Graded) const;
+	void ask(request Request);
+
+  private:
+	struct result
+	{
+		request asked;
+		profile::line line;
+		bool graded{false};
+	};
+	void work();
+
+	mutable std::mutex m_mutex;
+	std::condition_variable m_wake;
+	std::optional<request> m_asked;
+	std::optional<request> m_fitting;
+	std::optional<result> m_result;
+	bool m_stop{false};
+	std::thread m_thread;
+};
+
+// where Bed finds no ground, the heights lead from one end to the other. With a fitter the line it fitted for the same ground
+// is taken instead of fitting it again
+void follow(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading = {}, fitter const *Fitted = nullptr);
+// the same, though the line is fitted by the fitter: until it has it, the track gets the line it fitted last stretched over its
+// length, if it went on from the same end, or the heights from end to end
+void follow_meanwhile(std::vector<segment_data> &Pieces, ends const &Ends, bed_query const &Bed, grading const &Grading, fitter &Fitter);
 
 } // namespace ground_follow