Joining from federated networks to some larger rooms takes hours or even days.
Tried it a few months ago, was not impressed. I spent a couple days debugging it and asking for some support, but it turns out the official server software is by design incapable of joining large rooms instantly. It literally took 2 days for the room to appear in my chat client. And the message delivery was also delayed and far from instant.
To me personally IRC is way better than Matrix, when it comes to basic instant text messaging. For more advanced features I would rather use Discord, than spend days trying to get Matrix work.
Performance is going to be a major focus in 2022, with [fast federation room joins](https://matrix.org/blog/2021/12/22/the-mega-matrix-holiday-s...) and a new version of [the sync part of protocol](https://matrix.org/blog/2021/12/22/the-mega-matrix-holiday-s...).
In my opinion, there's no way Matrix can be a valid alternative, until it's as usable as the competitors.
On login, I was asked to validate from two devices. This is novel in this type of apps (at least, the mainstream ones), and it took me a while to figure it out.
Part of the confusion was also the fact that notifications about validations started to pile up, and I wasn't sure about I was doing.
I still don't know how I got the validation key (or however it's called) in this workflow.
Another novel concept is that, as far as I've understood, sessions are associated to keys, so, I suppose, if I lose a key, all the (encrypted) communications are gone. This would be acceptable, but the warning is very pushy; it's not clear to a new user if it's very strongly advised to enable encryption or not (will the other people trust me without?).
Communicating with other people is also confusing - I had two channels open with another user, and didn't know if any of it was valid.
Joining groups was confusing as well. I had a name (which was also strange; I still don't know if it's normal for the names to be "strange"), and again, couldn't figure out if I joined it or not (until I joined via other means).
This may be all necessary for a safe design, but definitely, it's a huge barrier to entry for somebody who expects a Slack-like experience.