On the other hand, I don't think anyone accused slack, with multi-megabyte webpages and even worse mobile app, of being a great experience. I think I used it once, and said no more.
I haven't had any real issues with desktop or mobile apps either.
Custom emojis are fun, inline markdown is useful, but it takes a lot of memory for something that, to me, seems like it should be lightweight. We've had IM since the 80's, after all. There's a part of me that has a visceral reaction to seeing an IM client taking more than 100mb of memory (though to be fair they seem to be getting improving that a lot.
Overall Slack is a net UX improvement over everything that came before.