--- a/apps/mobile/src/features/review/shikiReviewHighlighter.ts
+++ b/apps/mobile/src/features/review/shikiReviewHighlighter.ts
@@ -216,6 +216,35 @@
 
 function joinPatchLines(lines: ReadonlyArray<string>): string {
   return lines.map(stripTrailingNewline).join("\n");
+}
+
+// DIAGNOSTIC BUILD ONLY (never shipped, never in a prod A/B): records each JS slice of highlightLines
+// (work since the last resume, wait inside the yield) and logs one summary when the file finishes.
+type __T3DiagHl = { start: number; startDate: number; resumedAt: number; batches: number; slices: string[] };
+function __t3diagHlStart(): __T3DiagHl {
+  const now = performance.now();
+  return { start: now, startDate: Date.now(), resumedAt: now, batches: 0, slices: [] };
+}
+async function __t3diagHlYield(diag: __T3DiagHl, yieldFn: () => Promise<void>): Promise<void> {
+  const before = performance.now();
+  await yieldFn();
+  const after = performance.now();
+  diag.slices.push(`${(before - diag.resumedAt).toFixed(1)}/${(after - before).toFixed(1)}/${diag.batches}`);
+  diag.batches = 0;
+  diag.resumedAt = after;
+}
+function __t3diagHlEnd(diag: __T3DiagHl, lines: number, chars: number): void {
+  const end = performance.now();
+  diag.slices.push(`${(end - diag.resumedAt).toFixed(1)}/end/${diag.batches}`);
+  const work = diag.slices.map((s) => Number(s.split("/")[0]));
+  const waits = diag.slices.map((s) => s.split("/")[1]).filter((w) => w !== "end").map(Number);
+  const sum = (xs: number[]) => xs.reduce((a, b) => a + b, 0);
+  console.log(
+    `__t3diag hl summary lines=${lines} chars=${chars} slices=${diag.slices.length} total=${(end - diag.start).toFixed(1)}ms totalDate=${Date.now() - diag.startDate}ms work=${sum(work).toFixed(1)} waits=${sum(waits).toFixed(1)} maxWork=${Math.max(...work).toFixed(1)} maxWait=${waits.length ? Math.max(...waits).toFixed(1) : 0}`,
+  );
+  for (let i = 0; i < diag.slices.length; i += 20) {
+    console.log(`__t3diag hl slices[${i}] ${diag.slices.slice(i, i + 20).join(" ")}`);
+  }
 }
 
 function waitForNextFrame(): Promise<void> {
@@ -517,6 +546,7 @@
 
   const highlighter = await getHighlighter();
   const sourceLines = code.split("\n");
+  const __t3diag = __t3diagHlStart();
   const highlightedLines: Array<ReadonlyArray<ReviewHighlightedToken>> = [];
   // Batches resume from the previous batch's grammar state, so a comment or
   // template string that spans a batch boundary keeps its colors.
@@ -545,7 +575,8 @@
         end += 1;
       }
 
-      const tokenLines = highlighter.codeToTokensBase(sourceLines.slice(start, end).join("\n"), {
+      __t3diag.batches += 1;
+    const tokenLines = highlighter.codeToTokensBase(sourceLines.slice(start, end).join("\n"), {
         lang: language,
         theme,
         grammarState,
@@ -560,11 +591,12 @@
       start < sourceLines.length &&
       performance.now() - sliceStartedAt >= REVIEW_HIGHLIGHT_YIELD_AFTER_MS
     ) {
-      await waitForNextFrame();
+      await __t3diagHlYield(__t3diag, waitForNextFrame);
       sliceStartedAt = performance.now();
     }
   }
 
+  __t3diagHlEnd(__t3diag, sourceLines.length, code.length);
   return highlightedLines;
 }