Just did a quick check on my quadcore MBP, having Slack fullscreen out-of-view with nothing animated in view gives it a CPU usage of 0,1% with spikes up to 2,5%. Switch to a conversation with a single animated party parrot emoji, go to another fullscreen app once again, and CPU usage never drops below a whopping 22%. For an animated parrot cartoon.
I'm not sure if this is a bug in Slack or Electron. Doing the same in Chrome does not cause any issues, as GIFs seem to be suspended when the window is not in view. CPU usage goes down to almost 0 when you switch away from your GIF-filled Chrome window. For Slack, however, it barely changes. A big shame.
(On that note, I wonder how much power is wasted by party parrots via CPU usage. Where I work at least one is almost permanently visible in any conversation as a reaction emoji, so it's probably not insignificant.)