Same for me, I tried the latest Nightly a couple days ago to test the CoreAnimation work, and after 1 hour of use my Slack tab because so unbearably sluggish each keypress would take 1 second to render.

No extensions, no GIFs, no heavy tabs open, perhaps one or two open tabs.

No slowdowns at all after 8 hours on Slack w/ Safari or Brave. Firefox is definitely the worst browser on macOS, compared to the Windows version where it's absolutely perfect and always quick.