[meta] Improving Presence Performance

This issue has been migrated from [#9478](https://github.com/matrix-org/synapse/issues/9478).

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Presence is a feature of matrix that allows users to share their online/offline/unavailable status with other remote and local users, as well as present a status message. Traditionally this feature has been turned off in all major deployments due to its enormous performance costs. We'd like to turn it back on, and this issue is a collection of other issues that may help us get there.

There is currently no timeline for these tasks.

- https://github.com/matrix-org/synapse/issues/8956 ([fix #1](https://github.com/matrix-org/synapse/pull/9425))
- https://github.com/matrix-org/synapse/issues/8955
- https://github.com/matrix-org/synapse/issues/9415
- https://github.com/matrix-org/synapse/issues/9339
- ~~Moving presence work off of the master process~~ done in https://github.com/matrix-org/synapse/issues/9761