massive storm of presence EDUs when joining a large room

This issue has been migrated from [#3120](https://github.com/matrix-org/synapse/issues/3120).

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after you do your send_join to a room, all the servers in the room seem to immediately send you presence updates. Since this will often be the first contact you get from those servers, we then end up doing hundreds of server key requests, all of which end up starving out the actual join request - contribuiting to the join itself taking ages.

Start by tracing the send_join flow and the handling of incoming presence updates and server key requests. Determine how these requests compete with the join request, then verify that joining a large room no longer produces a storm of key requests that delays completion.

Written by the indexing model from the issue text.