I don't care that it's XML, but I do care that I can't send encrypted messages or make A/V calls to Matrix users simply because they choose to reinvent the existing standard.
Standardization is much more important than which encoding is popular right now.
But still, XML is easily compressed, parsers are much faster than required for IM and XMPP servers and clients are much more memory and CPU efficient, XML or not.
And it gets stranger. I looked into it more and found design goals stating that Matrix was originally designed as a way to link existing messaging systems like XMPP and Slack together. Yet, people remember it as its own thing, and that thing is a subset of what XMPP already offers.
When I looked into what it would take do commercial home servers, I found immature implementations that fall over while ejabberd has been battle tested on 2 million concurrent users on a single instance.
Best I can tell is that Matrix has better marketing. While Google is scaling back in XMPP for gtalk because of spam and lack of participation from other companies, Matrix is seizing mindshare as being newer and “better” and shinier. (And if they get big enough, it will also suffer from spam …)
Standardization is much more important than which encoding is popular right now.
But still, XML is easily compressed, parsers are much faster than required for IM and XMPP servers and clients are much more memory and CPU efficient, XML or not.