Sliding Sync: Increase concurrency of sliding sync a bit (#17696)

For initial requests a typical page size is 20 rooms, so we may as well
do the batching as 20.

This should speed up bigger syncs a little bit.

changelog.d/17696.misc

@@ -0,0 +1 @@
+Speed up sliding sync requests a bit where there are many room changes.

synapse/handlers/sliding_sync/__init__.py

@@ -267,7 +267,7 @@ async def handle_room(room_id: str) -> None:
 
         if relevant_rooms_to_send_map:
             with start_active_span("sliding_sync.generate_room_entries"):
-                await concurrently_execute(handle_room, relevant_rooms_to_send_map, 10)
+                await concurrently_execute(handle_room, relevant_rooms_to_send_map, 20)
 
         extensions = await self.extensions.get_extensions_response(
             sync_config=sync_config,

synapse/handlers/sliding_sync/extensions.py

@@ -38,6 +38,7 @@
     SlidingSyncConfig,
     SlidingSyncResult,
 )
+from synapse.util.async_helpers import concurrently_execute
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -534,7 +535,10 @@ async def get_receipts_extension_response(
             # For rooms we've previously sent down, but aren't up to date, we
             # need to use the from token from the room status.
             if previously_rooms:
-                for room_id, receipt_token in previously_rooms.items():
+                # Fetch any missing rooms concurrently.
+
+                async def handle_previously_room(room_id: str) -> None:
+                    receipt_token = previously_rooms[room_id]
                     # TODO: Limit the number of receipts we're about to send down
                     # for the room, if its too many we should TODO
                     previously_receipts = (
@@ -546,6 +550,10 @@ async def get_receipts_extension_response(
                     )
                     fetched_receipts.extend(previously_receipts)
 
+                await concurrently_execute(
+                    handle_previously_room, previously_rooms.keys(), 20
+                )
+
             if initial_rooms:
                 # We also always send down receipts for the current user.
                 user_receipts = (