Matrix/Riot does tend to break down in channels with 30k users all chatting, sending hundredthousands of messages per minute. (Which is a real use case of IRC).
Other networks have similar events.
And yes, most of the problems I describe only appear on Twitch, or other livestream chats.
"enormously. much of synapse's algorithms and DB schema are still unoptimised, plus python is not exactly renowned for being super-fast. we're effectively going through rewriting chunks of synapse - e.g. the new state storage representation in 0.18, and meanwhile Ruma is going through writing a cleanroom impl in Rust that should be a bajillion times faster :)"
(https://www.reddit.com/r/linux/comments/56xzyo/matrix_is_a_n...)
If that is a meaningful discussion is another topic, but many chats for live evenys, especially on Twitch, have these numbers of users and messages.
But even then, Matrix will likely not scale well enough to replace Twitch's backend, or to be able to nicely handle QuakeNet's event channels.
You'll also have to have a native client at that point, because the webclient also breaks down at those amounts. And you'll need to use a binary protocol or simplistic text protocol between client and server, because otherwise the user's mobile connection won't be able to handle that traffic.
Matrix will have to change a lot of things before that scaling is easy.
