Performance: Switch to PyPy + Latest Synapse

[nikhiljha]

Apparently Matrix runs faster on PyPy. We should switch out the Dockerfile to be based on the official PyPy image (pypy:slim-buster) or find one that's based on Alpine so it's more lightweight.

Also, the version of Synapse is a bit out of date. There have been some performance improvements, so we could take a look at that too.

[cg505]

i'm of the opinion that this should not be a blocker, but we should definitely work on it