add comment about possible future optimization

src/matrix/room/Room.js

@@ -76,6 +76,10 @@ export class Room extends BaseRoom {
             let eventsToDecrypt = roomResponse?.timeline?.events || [];
             // when new keys arrive, also see if any older events can now be retried to decrypt
             if (newKeys) {
+                // TODO: if a key is considered by roomEncryption.prepareDecryptAll to use for decryption,
+                // key.eventIds will be set. We could somehow try to reuse that work, but retrying also needs
+                // to happen if a key is not needed to decrypt this sync or there are indeed no encrypted messages
+                // in this sync at all.
                 retryEntries = await this._getSyncRetryDecryptEntries(newKeys, roomEncryption, txn);
                 if (retryEntries.length) {
                     log.set("retry", retryEntries.length);