# UNDF: UNDF-2026-000000729
--- a/apps/web/src/TextForEvent.tsx
+++ b/apps/web/src/TextForEvent.tsx
@@ -565,8 +565,10 @@ function textForPinnedEvent(event: MatrixEvent, client: MatrixClient, isTwelveHo
     const pinned = Array.isArray(content.pinned) ? content.pinned : [];
     const previouslyPinned: string[] = Array.isArray(prevContent.pinned) ? prevContent.pinned : [];
-    const newlyPinned = pinned.filter((item) => previouslyPinned.indexOf(item) < 0);
-    const newlyUnpinned = previouslyPinned.filter((item) => pinned.indexOf(item) < 0);
+    // Use Set for O(1) membership test; avoids O(P×Q) from indexOf inside filter
+    const pinnedSet = new Set(pinned);
+    const previouslyPinnedSet = new Set(previouslyPinned);
+    const newlyPinned = pinned.filter((item) => !previouslyPinnedSet.has(item));
+    const newlyUnpinned = previouslyPinned.filter((item) => !pinnedSet.has(item));