I disagree that the protocol is the problem. There's a combination of frontend/backend issues impacting performance, but servers written in faster languages don't seem to struggle with joining channels all that much.

Synapse being written in Python seems like a much bigger problem to me. This problem only exists in rooms not already available on the server, though; if you register with matrix.org then matrix.org rooms won't suffer nearly as much as running your own server.

Sadly, alternatives like Conduit still aren't fully-featured and they probably won't ever be as focus lies on the Element ecosystem. It's a real shame.