I haven't investigated too deeply. It may just be the clients rather than the server, but it's been there from the beginning. Just a slow laggy experience on all three of iOS, Debian and OSX with Element. Spinning progress bars, blank appearing rooms whilst stuff waits to load. Messages sometimes appearing in different orders for different people in the same room. Messages just not appearing for other people occasional. Random undecipherable complaints about encryption keys leading to some people not being able to read other peoples messages in the same room.

I regret getting people onto Matrix now. I'm keeping it running with the hope that things improve and because I don't want to get people to move to another system which may also have problems.

Should have just stuck with XMPP and my Prosody server, and IRC.