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Spawning from @kegsay [pointing out](https://matrix.to/#/!cnVVNLKqgUzNTOFQkz:matrix.org/$ExOO7J8uPUQSyH-9Uxc_QCa8jlXX9uK4VRtkSC0EI3o?via=element.io&via=matrix.org&via=jki.re) that the Sliding Sync endpoint doesn't handle a large room with a lot of state well on initial sync (requesting all state via `required_state: [ ["*","*"] ]`) (it just takes forever). After investigating further, the slow part is just `get_events_as_list(...)` fetching all of the current state ID's out for the room (which can be 100k+ events for rooms with a lot of membership). This is just a slow thing in Synapse in general and the same thing happens in Sync v2 or the `/state` endpoint. --- The only idea I had to improve things was to use `batch_iter` to only try fetching a fixed amount at a time instead of working with large maps, lists, and sets. This doesn't seem to have much effect though. There is already a `batch_iter(event_ids, 200)` in `_fetch_event_rows(...)` for when we actually have to touch the database and that's inside a queue to deduplicate work. I did notice one slight optimization to use `get_events_as_list(...)` directly instead of `get_events(...)`. `get_events(...)` just turns the result from `get_events_as_list(...)` into a dict and since we're just iterating over the events, we don't need the dict/map.
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Slight optimization when fetching state/events for Sliding Sync.
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