diff --git a/extension/src/content/detector.ts b/extension/src/content/detector.ts
index f369f0e..bfc19d8 100644
--- a/extension/src/content/detector.ts
+++ b/extension/src/content/detector.ts
@@ -93,9 +93,21 @@ setupFillListener();
 scan();
 
 // Watch for DOM changes (SPA navigation, dynamically loaded forms).
-const observer = new MutationObserver(() => {
-  scan();
-});
+// Plan C Phase 5: SPA churn fires the MutationObserver many times per
+// second. Trailing-edge debounce coalesces bursts so we run the full
+// scan() at most once per quiet 200ms window.
+const SCAN_DEBOUNCE_MS = 200;
+let scanTimer: ReturnType<typeof setTimeout> | undefined;
+
+function scheduleScan(): void {
+  if (scanTimer !== undefined) clearTimeout(scanTimer);
+  scanTimer = setTimeout(() => {
+    scanTimer = undefined;
+    scan();
+  }, SCAN_DEBOUNCE_MS);
+}
+
+const observer = new MutationObserver(scheduleScan);
 
 observer.observe(document.body, {
   childList: true,