diff --git a/docs/verb-reference.md b/docs/verb-reference.md
index 48de1e69..c820df2d 100644
--- a/docs/verb-reference.md
+++ b/docs/verb-reference.md
@@ -1379,13 +1379,13 @@ Not made by the test suite: this call needs more than the two-node headless sess
 
 ### `sim.faster`
 
-Double how much simulated time one tick covers, trading detail for how long a run takes to watch.
+Double how much simulated time one tick covers, which makes the run go twice as fast until the machine cannot keep up.
 
 **Takes** nothing.
 
-**Answers** `step_ms`. `step_ms` is the new tick length. Nothing bounds it: enough calls will step past whole transmissions.
+**Answers** `step_ms`. `step_ms` is the new tick length, capped at 1000. A tick is ten milliseconds of wall time and the engine still steps in its own 10 ms, so nothing is skipped: past what the machine can step in a tick the run simply goes as fast as it can, and the transport's realtime figure says how fast that is.
 
-**Example** - run twice as coarsely
+**Example** - run twice as fast
 
 ```json
 {"id":1,"method":"sim.faster","params":{}}
@@ -1551,9 +1551,9 @@ Halve how much simulated time one tick covers.
 
 **Takes** nothing.
 
-**Answers** `step_ms`. `step_ms` is the new tick length, which reaches zero after enough calls and stops the clock advancing at all.
+**Answers** `step_ms`. `step_ms` is the new tick length, never below 1. Below the engine's own 10 ms step, a tick steps only once it has been owed a whole step, so 5 is half real time.
 
-**Example** - run twice as finely
+**Example** - run at half the speed
 
 ```json
 {"id":1,"method":"sim.slower","params":{}}
@@ -1569,7 +1569,7 @@ Set how much simulated time one tick covers, which is the honest form of a speed
 
 | parameter | type | | what |
 |---|---|---|---|
-| `step_ms` | number | optional | simulated milliseconds per tick, which is what the engine actually paces on |
+| `step_ms` | number | optional | simulated milliseconds per tick, from 1 to 1000; a tick is ten milliseconds of wall time, so 10 is real time and 1000 is as fast as the machine can step |
 | `factor` | number | optional | a multiple of ten milliseconds per tick, read only when `step_ms` is absent, for scripts written against the old socket |
 
 **Answers** `step_ms`. `step_ms` is what it settled on. Neither parameter positive leaves the tick alone and reports it.
diff --git a/docs/verbs.json b/docs/verbs.json
index 38e15670..dc74085c 100644
--- a/docs/verbs.json
+++ b/docs/verbs.json
@@ -4709,14 +4709,14 @@
       }
     },
     "sim.faster": {
-      "what": "double how much simulated time one tick covers, trading detail for how long a run takes to watch",
+      "what": "double how much simulated time one tick covers, which makes the run go twice as fast until the machine cannot keep up",
       "returns": [
         "step_ms"
       ],
-      "answers": "`step_ms` is the new tick length. Nothing bounds it: enough calls will step past whole transmissions.",
+      "answers": "`step_ms` is the new tick length, capped at 1000. A tick is ten milliseconds of wall time and the engine still steps in its own 10 ms, so nothing is skipped: past what the machine can step in a tick the run simply goes as fast as it can, and the transport's realtime figure says how fast that is.",
       "example": {
         "params": {},
-        "what": "run twice as coarsely",
+        "what": "run twice as fast",
         "runnable": true
       }
     },
@@ -4867,10 +4867,10 @@
       "returns": [
         "step_ms"
       ],
-      "answers": "`step_ms` is the new tick length, which reaches zero after enough calls and stops the clock advancing at all.",
+      "answers": "`step_ms` is the new tick length, never below 1. Below the engine's own 10 ms step, a tick steps only once it has been owed a whole step, so 5 is half real time.",
       "example": {
         "params": {},
-        "what": "run twice as finely",
+        "what": "run at half the speed",
         "runnable": true
       }
     },
@@ -4880,7 +4880,7 @@
         {
           "name": "step_ms",
           "type": "number",
-          "what": "simulated milliseconds per tick, which is what the engine actually paces on"
+          "what": "simulated milliseconds per tick, from 1 to 1000; a tick is ten milliseconds of wall time, so 10 is real time and 1000 is as fast as the machine can step"
         },
         {
           "name": "factor",
diff --git a/internal/app/session/clockstep.go b/internal/app/session/clockstep.go
new file mode 100644
index 00000000..8860361f
--- /dev/null
+++ b/internal/app/session/clockstep.go
@@ -0,0 +1,51 @@
+package session
+
+import (
+	"context"
+
+	"github.com/MeshBench/meshbench/internal/app/state"
+)
+
+// stepTick advances the engine by one tick's worth of simulated time.
+//
+// A tick's worth, not one engine step. The engine's step is fixed at its own
+// 10 ms, and the tick used to take exactly one of them whatever the tick size
+// said - then set the clock back from the engine - so sim.faster, sim.slower
+// and sim.speed changed the number on the transport and nothing else: every
+// run in a session went at real time, with the processor mostly idle.
+//
+// So the tick is owed its size and pays it in whole engine steps. A tick
+// shorter than a step pays every few ticks, which is slower than real time; a
+// tick longer than the machine can cover in its 10 ms of wall time simply runs
+// as fast as the machine can go, which is what the lockstep clock exists to
+// allow: firmware reads simulated time, so nothing in it notices.
+//
+// Due sends are checked after every step rather than once a tick, so a send at
+// 20 s still goes at 20 s and not at the end of a 500 ms tick. The tick stops at
+// the end of a timed run, so sim.run lands where it was asked to.
+func (s *Sim) stepTick(w *state.World, tickMs uint32) {
+	// Close takes the engine away, from the workbench's own shutdown
+	// goroutine, while this tick stays installed - so a step arriving after a
+	// close finds nothing there and the panic takes the store's goroutine
+	// with it.
+	eng := s.eng
+	if eng == nil {
+		return
+	}
+	step := eng.Config.StepMs
+	if step == 0 {
+		step = 10
+	}
+	s.tickOwed += tickMs
+	for s.tickOwed >= step {
+		if until := w.RunUntilMs; until != 0 && eng.NowMs() >= until {
+			s.tickOwed = 0
+			return
+		}
+		_ = eng.Step(context.Background())
+		s.tickOwed -= step
+		// After the step, so a send lands on a mesh whose clock has already
+		// reached its moment.
+		s.fireDueSends(w)
+	}
+}
diff --git a/internal/app/session/clockstep_test.go b/internal/app/session/clockstep_test.go
new file mode 100644
index 00000000..920bda4a
--- /dev/null
+++ b/internal/app/session/clockstep_test.go
@@ -0,0 +1,56 @@
+package session
+
+import (
+	"testing"
+
+	"github.com/MeshBench/meshbench/internal/app/state"
+	"github.com/MeshBench/meshbench/internal/world/scenario"
+)
+
+// A tick covers the tick's size in simulated time, not one engine step. It
+// took one step whatever the speed control said, so "faster" relabelled the
+// transport and the run went at real time regardless.
+func TestATickCoversItsSize(t *testing.T) {
+	s := &Sim{gpuAsked: true}
+	s.build([]scenario.Node{repeaterNode("Alpha"), repeaterNode("Bravo")}, 869.618)
+	w := &state.World{}
+	before := s.eng.NowMs()
+	s.stepTick(w, 250)
+	if got := s.eng.NowMs() - before; got != 250 {
+		t.Fatalf("a 250 ms tick advanced the engine %d ms", got)
+	}
+}
+
+// A timed run ends where it was asked to, even when the tick is larger than
+// what is left of it.
+func TestATickStopsAtTheEndOfATimedRun(t *testing.T) {
+	s := &Sim{gpuAsked: true}
+	s.build([]scenario.Node{repeaterNode("Alpha"), repeaterNode("Bravo")}, 869.618)
+	w := &state.World{RunUntilMs: s.eng.NowMs() + 120}
+	s.stepTick(w, 1000)
+	if s.eng.NowMs() != w.RunUntilMs {
+		t.Fatalf("the run was to end at %d and the engine is at %d", w.RunUntilMs, s.eng.NowMs())
+	}
+	// And the next run does not inherit what the last tick was owed.
+	w.RunUntilMs = 0
+	before := s.eng.NowMs()
+	s.stepTick(w, 10)
+	if got := s.eng.NowMs() - before; got != 10 {
+		t.Fatalf("a 10 ms tick after a timed run advanced the engine %d ms", got)
+	}
+}
+
+// A tick shorter than an engine step steps only once it has been owed a whole
+// one: a 5 ms tick is half speed, not real time.
+func TestAShortTickStepsEveryOtherTime(t *testing.T) {
+	s := &Sim{gpuAsked: true}
+	s.build([]scenario.Node{repeaterNode("Alpha"), repeaterNode("Bravo")}, 869.618)
+	w := &state.World{}
+	before := s.eng.NowMs()
+	for i := 0; i < 4; i++ {
+		s.stepTick(w, 5)
+	}
+	if got := s.eng.NowMs() - before; got != 20 {
+		t.Fatalf("four 5 ms ticks advanced the engine %d ms, want 20", got)
+	}
+}
diff --git a/internal/app/session/engine.go b/internal/app/session/engine.go
index 8f075075..2cfcdb35 100644
--- a/internal/app/session/engine.go
+++ b/internal/app/session/engine.go
@@ -33,6 +33,9 @@ type Sim struct {
 	// sendClock is when each scheduled send last fired, so a repeating one
 	// repeats per interval rather than per tick.
 	sendClock sendClock
+	// tickOwed is simulated time a tick has been given and the engine has not
+	// yet stepped through, because a tick can be shorter than one step.
+	tickOwed uint32
 	// statesMu guards states, which Reflash writes from its own goroutine and
 	// the store goroutine reads to answer nodes.stats.
 	statesMu sync.Mutex
diff --git a/internal/app/session/simctl.verbs.json b/internal/app/session/simctl.verbs.json
index 7a0c7012..8b54d6fd 100644
--- a/internal/app/session/simctl.verbs.json
+++ b/internal/app/session/simctl.verbs.json
@@ -84,7 +84,7 @@
       {
         "name": "step_ms",
         "type": "number",
-        "what": "simulated milliseconds per tick, which is what the engine actually paces on"
+        "what": "simulated milliseconds per tick, from 1 to 1000; a tick is ten milliseconds of wall time, so 10 is real time and 1000 is as fast as the machine can step"
       },
       {
         "name": "factor",
diff --git a/internal/app/session/verbs.go b/internal/app/session/verbs.go
index fb88fb86..c47ec48d 100644
--- a/internal/app/session/verbs.go
+++ b/internal/app/session/verbs.go
@@ -174,25 +174,14 @@ func Register(st *state.Store, s *Sim) {
 		for i, n := range f.nodes {
 			index[n.Name] = i
 		}
-		w.Tick = func(uint32) {
+		w.Tick = func(tickMs uint32) {
 			w.FirmwareRunning, w.FirmwareStarting = s.firmwareCount(), s.starting.Load()
 			// A sweep cell paces its own engine against wall time and counts on
 			// that pacing. Stepping it again here would not make the run
 			// faster - it would make its airtime accounting wrong - so while a
 			// cell owns the clock this only reads.
 			if !s.benchOwnsTheClock() {
-				// Close takes the engine away, from the workbench's own
-				// shutdown goroutine, while this tick stays installed - so a
-				// step arriving after a close finds nothing there and the
-				// panic takes the store's goroutine with it. fireDueSends
-				// below has always asked; this is the call that did not.
-				if eng := s.eng; eng != nil {
-					_ = eng.Step(context.Background())
-				}
-				// Whatever the schedule is due to say at this moment of
-				// simulated time. After the step, so a send lands on a mesh
-				// whose clock has already reached its moment.
-				s.fireDueSends(w)
+				s.stepTick(w, tickMs)
 			}
 			// A rebuild anywhere leaves the matrix cold; this is the one
 			// place every run passes through, so it is where the warm is
diff --git a/internal/app/session/verbsclock.verbs.json b/internal/app/session/verbsclock.verbs.json
index b27dd8f5..d9ad3d3f 100644
--- a/internal/app/session/verbsclock.verbs.json
+++ b/internal/app/session/verbsclock.verbs.json
@@ -1,13 +1,13 @@
 {
   "sim.faster": {
-    "what": "double how much simulated time one tick covers, trading detail for how long a run takes to watch",
+    "what": "double how much simulated time one tick covers, which makes the run go twice as fast until the machine cannot keep up",
     "returns": [
       "step_ms"
     ],
-    "answers": "`step_ms` is the new tick length. Nothing bounds it: enough calls will step past whole transmissions.",
+    "answers": "`step_ms` is the new tick length, capped at 1000. A tick is ten milliseconds of wall time and the engine still steps in its own 10 ms, so nothing is skipped: past what the machine can step in a tick the run simply goes as fast as it can, and the transport's realtime figure says how fast that is.",
     "example": {
       "params": {},
-      "what": "run twice as coarsely",
+      "what": "run twice as fast",
       "runnable": true
     }
   },
@@ -39,10 +39,10 @@
     "returns": [
       "step_ms"
     ],
-    "answers": "`step_ms` is the new tick length, which reaches zero after enough calls and stops the clock advancing at all.",
+    "answers": "`step_ms` is the new tick length, never below 1. Below the engine's own 10 ms step, a tick steps only once it has been owed a whole step, so 5 is half real time.",
     "example": {
       "params": {},
-      "what": "run twice as finely",
+      "what": "run at half the speed",
       "runnable": true
     }
   },