perf(desktop): cap aux backfill request concurrency

Finding L3-3: fetchChunkedAuxEvents awaited each auxiliary-event chunk sequentially, delaying visible reaction/edit/deletion backfills on large history windows.

Run aux chunks through a small shared concurrency helper so relay history requests overlap without unbounded fanout. The helper preserves chunk result order and the existing fail-fast semantics.

Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>

desktop/src/shared/api/concurrency.ts

@@ -0,0 +1,20 @@
+export async function collectWithConcurrency<T, R>(
+  items: T[],
+  concurrency: number,
+  worker: (item: T) => Promise<R>,
+): Promise<R[]> {
+  const workerCount = Math.min(Math.max(1, concurrency), items.length);
+  const results = new Array<R>(items.length);
+  let nextIndex = 0;
+
+  await Promise.all(
+    Array.from({ length: workerCount }, async () => {
+      while (nextIndex < items.length) {
+        const currentIndex = nextIndex++;
+        results[currentIndex] = await worker(items[currentIndex]);
+      }
+    }),
+  );
+
+  return results;
+}

desktop/src/shared/api/relayClientSession.ts

@@ -29,6 +29,7 @@ import {
   buildChannelMentionFilter,
   buildGlobalStreamFilter,
 } from "@/shared/api/relayChannelFilters";
+import { collectWithConcurrency } from "@/shared/api/concurrency";
 import { replayLiveSubscriptions } from "@/shared/api/relayReconnectReplay";
 import { RelayConnectionStateEmitter } from "@/shared/api/relayConnectionStateEmitter";
 import {
@@ -41,7 +42,8 @@ import { buildThreadReferenceTags } from "@/features/messages/lib/threading";
 
 const RECONNECT_BASE_DELAY_MS = 1_000,
   RECONNECT_MAX_DELAY_MS = 30_000,
-  EVENT_BATCH_MS = 16;
+  EVENT_BATCH_MS = 16,
+  AUX_BACKFILL_CONCURRENCY = 4;
 
 /**
  * Passive liveness check. The relay sends heartbeat pings every 30s; if no
@@ -217,10 +219,11 @@ export class RelayClient {
       chunks.push(eventIds.slice(i, i + AUX_BACKFILL_CHUNK_SIZE));
     }
 
-    const batches: RelayEvent[][] = [];
-    for (const ids of chunks) {
-      batches.push(await this.requestHistory(buildFilter(channelId, ids)));
-    }
+    const batches = await collectWithConcurrency(
+      chunks,
+      AUX_BACKFILL_CONCURRENCY,
+      (ids) => this.requestHistory(buildFilter(channelId, ids)),
+    );
 
     return batches.flat();
   }