Matrix.org is so slow it's borderline unusable, that's right. However, while switching to another homeserver (and avoiding federating with big rooms like Matrix HQ) helps a lot, Riot isn't exactly a lightweight client as well.

Agreed, I set one up and tried to convince people to switch but the latency made it just unusable. It worked fine with a channel or two but if someone tried to join a federated channel it would bring the whole thing to its knees for hours at a time, knocking out the local channels with it.