[BUG] Performance issues with v1.3.0a1

[nexy7574]

Describe the bug
After 1.2's release, and shortly before 1.3.0a1's release, I noticed some performance slowdowns in one of my bots, where event processing was taking 5-15 seconds for some reason. As I could only reproduce this with this one bot, I just put it down to an issue with my code and moved on.

However, after the release of 1.3.0a1, and updating my other bots to it, I've noticed they are suffering from the same issue - sporadically experiencing latencies of 5 or more seconds per sync.

I have done extensive debugging of this, however cannot currently pinpoint a source. I initially suspected the sync store to be the problem, but this issue had existed even before the rewrite, so I suspect that it is somewhere else.

To Reproduce
Run a bot with a ping command similar to this one for example, join a few rooms (in my case I did some exclusive testing in #test:envs.net, #test_enc:envs.net, #offtopic:nexy7574.co.uk, and #niobot-test:nexy7574.co.uk), and run a few pings. The first few will seemingly respond fine, but eventually, they will arbitrarily start taking seconds to respond.

Expected behavior
Latency (locally, not over federation, on a non-overloaded high performance server) should not be a magnitude of seconds.

Additional context
The issue is easily detectable in logs as there is a large delay between sync() calls when in the sync_forever() loop. Assuming the "why is my bot not responding"