The desktop clients are built on Electron [1]. I've been using it for about 3 weeks and there's been a few times where performance issues were noticeable but not bad enough for me to start digging in the code to find the issue. Overall I think it's a well put together app, but people who hate anything built on Electron will probably have bad things to say about it.
Slack's performance also depends a ton on how much your teams are using it, and how many teams you're on.
"reactjis" are one of the worst offenders -- I've had a channel open with some people messing around with them, and it'll be casually consuming 30% of cpu and 3 gigs of ram on my top-of-the-line macbook.
I would have long since deleted an IRC client that was such a poor performer, but unfortunately there's not any good/complete 3rd party slack clients.
Lots of poorly written webapps give Electron a bad name. Slack, unfortunately does have it's performance issues, but there are other popular chat apps written in Electron that are very performant, even on large teams.
[1] https://github.com/mattermost/desktop