diff --git a/index.html b/index.html
index 4479db2..b18a832 100644
--- a/index.html
+++ b/index.html
@@ -91,7 +91,7 @@
     <div class="panel">
       <h2>stabilised (head-local)</h2>
       <canvas id="cv-stab"></canvas>
-      <div class="legend">should sit still except the mouth · grey = held plate outline</div>
+      <div class="legend">should sit still except the mouth · grey = held plate outline<br>dark green ghost = unshifted mouth when lead ≠ 0</div>
     </div>
     <div class="panel">
       <h2>flat render — 320×200 indexed</h2>
diff --git a/js/app.js b/js/app.js
index 673a56f..752bac5 100644
--- a/js/app.js
+++ b/js/app.js
@@ -1,6 +1,6 @@
 import { FaceLandmarker, FilesetResolver } from 'https://cdn.jsdelivr.net/npm/@mediapipe/tasks-vision@1.0.1/vision_bundle.mjs';
 import { LIPS_OUTER, LIPS_INNER, FACE_OVAL } from './landmarks.js';
-import { stabilize, toRasterRing, smoothContours, suggestPlateFrames, heldFrame } from './pipeline.js';
+import { stabilize, toRasterRing, smoothContours, suggestPlateFrames, heldFrame, shiftIndex } from './pipeline.js';
 import { IndexedRaster } from './raster.js';
 import { drawRegistered, posterizeInto } from './underlay.js';
 import { extractTeeth } from './interior.js';
@@ -26,6 +26,7 @@ const state = {
   frame: 0, playing: false, faceBox: null,
   fps: 12, audio: null,     // fps comes from manifest.json, never guessed
   aspect: 1,                // imgW/imgH; converts MediaPipe's anisotropic space
+  lead: 0,                  // performance-track offset in frames
   interior: null,           // per-frame teeth measurement from image content
   teeth: null,              // resolved per-frame {show, t} after knobs
 };
@@ -165,6 +166,7 @@ function makeXform(stab, neutral) {
 function rebuild(resetKeep) {
   if (!state.dense) return;
   const o = opts();
+  state.lead = o.lead;
   const N = state.dense.length;
 
   state.stab = stabilize(state.dense, o.smoothWin, state.aspect);
@@ -345,8 +347,9 @@ const keptSorted = () => [...state.keep].sort((a, b) => a - b);
 // Positive lead = the mouth arrives earlier. Only performance parts shift; the
 // head stays with the audio, because it is the mouth that should anticipate.
 function leadIndex(f) {
-  const N = state.dense.length;
-  return Math.min(N - 1, Math.max(0, f + opts().lead));
+  // Reads a cached scalar, not opts(): this runs once per strip thumbnail, and
+  // calling opts() here meant ~14 DOM reads x 74 frames on every redraw.
+  return shiftIndex(f, state.lead, state.dense.length);
 }
 
 function blit(canvas, raster, zoom) {
@@ -364,7 +367,7 @@ function drawAll() {
 function drawReadout() {
   const kept = keptSorted();
   const runs = kept.map((k, i) => (i + 1 < kept.length ? kept[i + 1] : state.dense.length) - k);
-  const lead = opts().lead;
+  const lead = state.lead;
   const teethFrames = state.teeth ? state.teeth.filter((t) => t.show).length : 0;
   el('readout').textContent =
     `${state.dense.length} frames → ${kept.length} drawings · ` +
@@ -378,10 +381,13 @@ function drawReadout() {
 function drawPanes() {
   const f = state.frame, kept = keptSorted();
   const pf = heldFrame(kept, f);
-  const lead = opts().lead;
+  // The mouth frame is always shown, not only when shifted, so the number can be
+  // watched diverging from f rather than taken on trust.
+  const lead = state.lead;
   el('framelabel').textContent =
-    `f ${f} / ${state.dense.length - 1} · ${(f / state.fps).toFixed(2)}s · plate f${pf}` +
-    (lead ? ` · mouth f${leadIndex(f)} (${lead > 0 ? '+' : ''}${lead})` : '') +
+    `f ${f} / ${state.dense.length - 1} · ${(f / state.fps).toFixed(2)}s · ` +
+    `plate f${pf} · mouth f${leadIndex(f)}` +
+    (lead ? ` (${lead > 0 ? '+' : ''}${lead} = ${(lead / state.fps * 1000).toFixed(0)}ms)` : '') +
     (state.keep.has(f) ? ' · KEPT' : ' · held');
 
   const c1 = el('cv-source'), g1 = c1.getContext('2d');
@@ -415,6 +421,9 @@ function drawPanes() {
   const z = (pts) => pts.map((p) => ({ x: p.x * ZOOM, y: p.y * ZOOM }));
   const mf = leadIndex(f);
   strokePts(g2, z(state.plates[pf]), '#3b4a63');
+  // With a lead set, the unshifted contour is drawn as a ghost so the offset is
+  // something you can see rather than something you have to trust.
+  if (mf !== f) strokePts(g2, z(state.outer[f]), '#2f6b46');
   strokePts(g2, z(state.outer[mf]), '#4ade80');
   if (!state.hidden[mf]) strokePts(g2, z(state.inner[mf]), '#f87171');
 
@@ -611,6 +620,7 @@ function runSynthetic() {
   attachAudio(null);
   state.fps = 12;
   state.aspect = 1;         // synthetic landmarks are generated square
+  state.lead = 0;
   state.interior = null;    // no pixels, so no teeth
 
   state.dense = synthDense(72);
@@ -628,6 +638,7 @@ for (const id of ['verts', 'smoothWin', 'contourSmooth', 'apertureThresh', 'tol'
       ? (+el(id).value / 1000).toFixed(3)
       : ['teethOn', 'teethErode', 'tongueReject', 'topBias'].includes(id)
         ? (+el(id).value / 100).toFixed(2)
+      : id === 'lead' && +el(id).value > 0 ? `+${el(id).value}`
       : el(id).value;
     if (id === 'tol') return;                 // tol only matters when you ask for a suggestion
     rebuild(false);
@@ -699,7 +710,8 @@ window.addEventListener('keydown', (e) => {
   } else if (e.key === '[' || e.key === ']') {
     const n = el('lead');
     n.value = Math.max(+n.min, Math.min(+n.max, +n.value + (e.key === ']' ? 1 : -1)));
-    el('leadv').textContent = n.value;
+    state.lead = +n.value;
+    el('leadv').textContent = state.lead > 0 ? `+${state.lead}` : String(state.lead);
     drawAll();
   } else if (e.key === 'b') {
     const sel = el('plateMode');
diff --git a/js/pipeline.js b/js/pipeline.js
index d75ea99..a3b8d2f 100644
--- a/js/pipeline.js
+++ b/js/pipeline.js
@@ -177,3 +177,12 @@ export function heldFrame(kept, f) {
   for (const k of kept) { if (k <= f) hit = k; else break; }
   return hit;
 }
+
+// Shift a performance track against the clock, clamped at the ends.
+//
+// Pure and exported so the shift can actually be asserted: "the slider feels
+// like it does nothing" is otherwise indistinguishable from "the slider does
+// nothing", and at 24fps a lead of 1 is 42ms, which is small enough to doubt.
+export function shiftIndex(f, lead, n) {
+  return Math.min(n - 1, Math.max(0, f + lead));
+}
diff --git a/js/selftest.js b/js/selftest.js
index 82abc7f..dabbc3f 100644
--- a/js/selftest.js
+++ b/js/selftest.js
@@ -9,7 +9,7 @@
 
 import { LIPS_OUTER, LIPS_INNER, FACE_OVAL, RIGID, subsampleSlots, subsampleRing } from './landmarks.js';
 import { fitSimilarity, applySim, procrustesMean, smoothTransforms } from './mathutil.js';
-import { stabilize, toRasterRing, selectKeys, activeKey } from './pipeline.js';
+import { stabilize, toRasterRing, selectKeys, activeKey, shiftIndex } from './pipeline.js';
 import { IndexedRaster, hexToRgb } from './raster.js';
 import { writeTake } from './take.js';
 import { otsuForTest, scaleRing } from './interior.js';
@@ -206,6 +206,31 @@ export function run() {
   ok('activeKey holds between keys',
      activeKey(sel.keys, sel.keys[1].f - 1).f === sel.keys[0].f);
 
+  // mouth lead: a shift that "feels like it does nothing" is indistinguishable
+  // from one that does nothing, so assert the arithmetic directly.
+  ok('lead 0 is identity', [0, 5, 71].every((f) => shiftIndex(f, 0, 72) === f));
+  ok('positive lead moves the source frame forward', shiftIndex(10, 2, 72) === 12);
+  ok('negative lead moves it back', shiftIndex(10, -3, 72) === 7);
+  ok('lead clamps at the start', shiftIndex(1, -6, 72) === 0);
+  ok('lead clamps at the end', shiftIndex(70, 6, 72) === 71);
+  {
+    // ...and that it selects different POSES, not merely different indices.
+    // Checked across the whole track rather than at one pair: synthetic poses
+    // hold for nine-frame beats, so any single pair can legitimately be
+    // identical while the shift works perfectly.
+    const N = shapes.length;
+    let moved = 0, total = 0;
+    for (let f = 0; f < N; f++) {
+      const a = shapes[shiftIndex(f, 0, N)], b = shapes[shiftIndex(f, 3, N)];
+      let d = 0;
+      for (let i = 0; i < a.length; i++) d += Math.hypot(a[i].x - b[i].x, a[i].y - b[i].y);
+      total++;
+      if (d / a.length > 0.5) moved++;
+    }
+    ok('a lead of 3 changes the pose on a good share of frames', moved / total > 0.2,
+       `${moved}/${total} frames differ`);
+  }
+
   // rasteriser: indexed, hard-edged, no blending
   const r = new IndexedRaster(64, 48);
   r.clear(0);