Generate room sync data concurrently (#17458)

This is also what we do for standard `/sync`.

changelog.d/17458.misc

@@ -0,0 +1 @@
+Speed up generating sliding sync responses.

synapse/handlers/sliding_sync.py

@@ -28,6 +28,7 @@
 from synapse.events import EventBase
 from synapse.events.utils import strip_event
 from synapse.handlers.relations import BundledAggregations
+from synapse.logging.opentracing import start_active_span, tag_args, trace
 from synapse.storage.databases.main.roommember import extract_heroes_from_room_summary
 from synapse.storage.databases.main.stream import CurrentStateDeltaMembership
 from synapse.storage.roommember import MemberSummary
@@ -43,6 +44,7 @@
 )
 from synapse.types.handlers import OperationType, SlidingSyncConfig, SlidingSyncResult
 from synapse.types.state import StateFilter
+from synapse.util.async_helpers import concurrently_execute
 from synapse.visibility import filter_events_for_client
 
 if TYPE_CHECKING:
@@ -592,11 +594,14 @@ async def current_sync_for_user(
 
         # Fetch room data
         rooms: Dict[str, SlidingSyncResult.RoomResult] = {}
-        for room_id, room_sync_config in relevant_room_map.items():
+
+        @trace
+        @tag_args
+        async def handle_room(room_id: str) -> None:
             room_sync_result = await self.get_room_sync_data(
                 user=sync_config.user,
                 room_id=room_id,
-                room_sync_config=room_sync_config,
+                room_sync_config=relevant_room_map[room_id],
                 room_membership_for_user_at_to_token=room_membership_for_user_map[
                     room_id
                 ],
@@ -606,6 +611,9 @@ async def current_sync_for_user(
 
             rooms[room_id] = room_sync_result
 
+        with start_active_span("sliding_sync.generate_room_entries"):
+            await concurrently_execute(handle_room, relevant_room_map, 10)
+
         extensions = await self.get_extensions_response(
             sync_config=sync_config, to_token=to_token
         )