[1_player]

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.

[Caspy7]

I know about the CoreAnimation work. Hadn't heard about this accumulated sluggishness that you describe. Have you tried safe mode? A "Refresh"?

[1_player]

It was an installation from scratch, no leftover files or configs. And I actually did a manual refresh in the preferences.