p=1 req taking 4s to return for large accounts

[ara4n]

while looking into element-hq/element-x-ios#2062, @kegsay noted that the p=1 req for my acct took 4s to return, which feels worryingly long (especially if the app is incorrectly blocking the UI waiting for it to return).

[kegsay]

The bulk of the time was spent inside RoomStateAfterEventPosition for 100 all_rooms, 4.53/4.8s.

[kegsay]

"required_state":[["m.room.avatar",""],["m.room.encryption",""],["m.room.member","$LAZY"],["m.room.power_levels",""]],"timeline_limit":1

Unfortunately, $LAZY means we would need to query membership_events array for each room :(

[kegsay]

I wonder if we can optimise just that particular query, i.e cases where:

• you want lazy members
• timeline limit = 1

In that case, it might be quicker to just guess from the events table e.g SELECT event_nid FROM syncv3_events WHERE event_type='m.room.member' AND state_key='@sender' AND room_id='!theroom' AND (membership='join' OR membership='_join'). Then do a simple existence check in the membership_events array. If it is in the array, great we saved a ton of time. If it isn't, boo, we still need to check as we may have guessed the wrong membership event (e.g a join from months ago).