Haven't noticed any issue with responsiveness with the official Slack client but my boss was kind enough to set me up with a pretty beefy Macbook Pro with more RAM than I'd ever need to use.
It may be an issue with their linux client (pretty much all developers I work with are on linux), though I have heard complaints from windows users as well (but it's windows and the users are from the design team, so who knows what their computers do).