doc(event cache): add a code comment indicating why room updates processing isn't happening concurrently

Author: bnjbvr

crates/matrix-sdk/src/event_cache/mod.rs

@@ -525,6 +525,11 @@ impl EventCacheInner {
             self.multiple_room_updates_lock.lock().await
         };
 
+        // Note: bnjbvr tried to make this concurrent at some point, but it turned out
+        // to be a performance regression, even for large sync updates. Lacking
+        // time to investigate, this code remains sequential for now. See also
+        // https://github.com/matrix-org/matrix-rust-sdk/pull/5426.
+
         // Left rooms.
         for (room_id, left_room_update) in updates.left {
             let room = self.for_room(&room_id).await?;