It’s not decentralized so somebody’s server has to take the hit even if the protocol is about as barebones as it gets. You’re also missing some basic features like usernames & rooms having collisions—heck usernames & rooms can’t even contain Unicode which hurts accessibility.
Google Chat on my 2008 Blackberry (XMPP client) was the fastest IM experience I've ever had. I finally understood the meaning of "Crackberry" when I had that setup. The combination of the instantaneous speed and keyboard was something I haven't experienced since.
Matrix Homeservers like Synapse can run on anything now, and there is conduit which should be lighter too.
Last I checked, the way the protocol works still causes a huge amount of load for minimal usage. Because of the way events need to propagate and the types of events that propagate
I haven't seen this in my servers.
I host three matrix servers and so far synapse isn't out of the ordinary.
Matrix is not lightweight or simple.
Matrix shifts much of the burden to the server so that clients can be simpler (except for e2ee, which has to be done clientside).
Which if true is still a burden that prevents simple self-hosting. You can (& I did briefly) install an XMPP server on a 10-year-old smart phone with postmarketOS on your network & have a reasonable experience.