[meta] Improving Presence Performance

[matrixbot]

This issue has been migrated from #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.

• Presence logic for handling newly joined rooms is very inefficient and does duplicate work matrix-org/synapse#8956 (fix #1)
• Synchrotrons always wake up sync streams for presence updates matrix-org/synapse#8955
• Not all presence updates should be sent out over federation immediately. matrix-org/synapse#9415
• Stop sending unneeded presence messages matrix-org/synapse#9339
• Moving presence work off of the master process done in Move presence off master matrix-org/synapse#9761