I made the opposite experiece and just made a test with some medium and similar sized chat servers with many users amd history (bitcoin related chat servers) on Slack and Discord in Chrome which should represent their native Electron apps:

Data from Chrome Inspector's network tab:

          Slack Discord
  Req.       93     103
  Transf.22.4MB   5.2MB
  Finish 39.19s   6.48s
  DomCL.  6.53s   4.01s
  Load   21.70s   4.16s

Maybe some more of us could do some tests as well. Anyways, I really can't believe that Discord is sluggish. Slack definitely is. First I thought it is so slow because they load all the chat history but they don't.