Investigate if `uvloop` with `asyncioreactor` gives some speedboosts

This issue has been migrated from [#10366](https://github.com/matrix-org/synapse/issues/10366).

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From the [twisted 21.7.0.rc1 release](https://github.com/twisted/twisted/releases/tag/twisted-21.7.0rc1):
> twisted.internet.asyncioreactor.AsyncioSelectorReactor will no longer raise a TypeError like "SelectorEventLoop required, instead got: " (broken since 21.2.0).

[`uvloop`](https://uvloop.readthedocs.io/) is [pretty fast](http://magic.io/blog/uvloop-blazing-fast-python-networking/), and twisted can utilize asyncio event loops with `asyncioreactor`, so i wonder if some speed boosts could be gained by switching to `asyncioreactor` + `uvloop`, even if optionally.