diff --git a/frontend/src/lib/timeline/YearBand.svelte b/frontend/src/lib/timeline/YearBand.svelte
index ef4579d2..2783d141 100644
--- a/frontend/src/lib/timeline/YearBand.svelte
+++ b/frontend/src/lib/timeline/YearBand.svelte
@@ -54,11 +54,15 @@ const split = $derived(
 );
 const loose = $derived(split.loose);
 const dense = $derived(isDense(loose.length));
+// Clusters keyed by eventId (built once in splitYearLetters): row assembly looks a letter's
+// disposition up in O(1) — `byEvent.has(linkedEventId)` — instead of scanning the loose array
+// per letter (was O(L²) on a dense band), and resolves an event's card with `byEvent.get`.
+const byEvent = $derived(split.byEvent);
 
 const rows = $derived.by<Row[]>(() => {
 	const out: Row[] = [];
 	const { clusters } = split;
-	const consumed: string[] = [];
+	const consumed: Record<string, true> = {};
 	let stripInserted = false;
 	let letterIndex = 0;
 
@@ -67,16 +71,19 @@ const rows = $derived.by<Row[]>(() => {
 			// A curated event whose letters live in THIS band becomes the contained card's
 			// header — its title reads once, no separate pill (REQ-002). Otherwise it stays a
 			// plain pill/world-band (REQ-005).
-			const cluster = entry.eventId ? clusters.find((c) => c.eventId === entry.eventId) : undefined;
+			const cluster = entry.eventId ? byEvent.get(entry.eventId) : undefined;
 			if (cluster) {
 				out.push({ t: 'eventcard', event: entry, cluster });
-				consumed.push(cluster.eventId);
+				consumed[cluster.eventId] = true;
 			} else {
 				out.push({ t: 'event', entry });
 			}
-		} else if (loose.includes(entry)) {
-			// A loose letter: alternate while sparse, or fold the whole loose set into one
-			// density strip (inserted once, at the first loose letter) when dense.
+		} else if (
+			entry.kind === 'LETTER' &&
+			!(entry.linkedEventId && byEvent.has(entry.linkedEventId))
+		) {
+			// A loose letter (not clustered): alternate while sparse, or fold the whole loose set
+			// into one density strip (inserted once, at the first loose letter) when dense.
 			if (!dense) {
 				out.push({ t: 'letter', entry, side: letterIndex % 2 === 0 ? 'left' : 'right' });
 				letterIndex += 1;
@@ -91,7 +98,7 @@ const rows = $derived.by<Row[]>(() => {
 	// Cross-year clusters: a cluster whose event is NOT a same-year EVENT entry renders as a
 	// text-header card (no pill, no edit link) holding this year's linked letters (REQ-004).
 	for (const cluster of clusters) {
-		if (!consumed.includes(cluster.eventId)) {
+		if (!consumed[cluster.eventId]) {
 			out.push({ t: 'eventcard', cluster });
 		}
 	}
diff --git a/frontend/src/lib/timeline/eventClustering.ts b/frontend/src/lib/timeline/eventClustering.ts
index e80a43fd..212c7338 100644
--- a/frontend/src/lib/timeline/eventClustering.ts
+++ b/frontend/src/lib/timeline/eventClustering.ts
@@ -19,6 +19,9 @@ export interface EventCluster {
 export interface SplitLetters {
 	clusters: EventCluster[];
 	loose: TimelineEntryDTO[];
+	/** Clusters keyed by `eventId` for O(1) lookup during row assembly (a letter's disposition is
+	 *  `byEvent.has(linkedEventId)`; an event's card is `byEvent.get(eventId)`). */
+	byEvent: Map<string, EventCluster>;
 }
 
 /**
@@ -81,5 +84,5 @@ export function splitYearLetters(
 		}
 	}
 
-	return { clusters, loose };
+	return { clusters, loose, byEvent };
 }