Internet forums are disappearing because now everything is Reddit and Discord. And that's worrying.
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Matrix is notorious for its poor performance with large/numerous groups. They keep claiming to improve it, but it's still bad.
I mean, it's great that it works for you, but be honest: isn't your tolerance for technological friction a bit higher than the average bear's? People complain that Mastodon is too hard, and Matrix is ten times worse to sign up for and use.
I hate to say it, but Matrix is never going to be mainstream. Its UX is bad and it seems like it's too bloated to fix. If I tried to get people to move from Discord to Matrix, they'd never take me seriously again. It was hard enough getting people to move from Facebook Messenger to Signal.