Initial /sync when filtered to a single room can be very slow

[matrixbot]

This issue has been migrated from #10842.

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Description

The /rooms/{roomId}/initialSync endpoint is deprecated and the documentation suggests using /sync instead.

However, using /sync with a room filter has drastically worse performance for users in very many rooms,
eg. taking up to 400 seconds for a user in 2,200 rooms and pulling 100,000+ events from the database.

Steps to reproduce

GET /_matrix/client/r0/sync?filter=%7B%22room%22:+%7B%22rooms%22:+%5B%22!XXXXXXXXXXXXXXXXXX:matrix.org%22%5D,+%22state%22:+%7B%22rooms%22:+%5B%22!XXXXXXXXXXXXXXXXXX:matrix.org%22%5D%7D,+%22timeline%22:+%7B%22rooms%22:+%5B%22!XXXXXXXXXXXXXXXXXX:matrix.org%22%5D,+%22senders%22:+%5B%22@xxxxxxxxxx:matrix.org%22%5D%7D,+%22ephemeral%22:+%7B%22limit%22:+0,+%22types%22:+%5B%5D%7D,+%22account_data%22:+%7B%22limit%22:+0,+%22types%22:+%5B%5D%7D%7D,+%22presence%22:+%7B%22limit%22:+0,+%22types%22:+%5B%5D%7D,+%22account_data%22:+%7B%22limit%22:+0,+%22types%22:+%5B%5D%7D%7D

Version information

• Homeserver: matrix.org