Have you tried it with a different homeserver (either hosted or some other community server)? Matrix.org has a fucktonne of users (~5 million from memory), and given how little funding they have it's shocking that they can afford to run a homeserver at all. I've run my own homeserver for the past few years and I haven't ever had any issues with performance (even when talking to people on Matrix.org).

I can attest to that. I run a Matrix homeserver for a small community, and it's blazingly fast all the time.

Everyone should migrate off matrix.org sooner rather than later. I fully expect seeing regional public Matrix homeservers popping up here and there eventually, just like regional public Jabber servers did back in the day.

Or, indeed like public e-mail providers did.

That's not my experience at all. It got better when I left some large rooms. But I still regularly get things like messages that the server is offline, taking a minute before the UI shows that my message was sent, after my laptop was suspended, it takes minutes before element becomes usable again. I run my own IRC bridge, and if they send more than 5 messages a minute I start losing messages and they get out of order.