I found a post on Reddit that has a possible solution how to rollback on a mac to Patch 6 before the problems occurred. Scroll down to tenkitron's post.
https://www.reddit.com/r/macgaming/comments/1bag2b5/baldurs_gate_3_fps_drop/I haven't tested it yet, still waiting for the depot download to complete, but may be worth a shot if you don't want to wait for any incoming fix.
EDITED TO ADD:
It works! Any saves made after that patch are no longer compatible, which was expected, but the rollback brought my FPS back up where it was. Worth it if you don't want to wait for Larian to get this fixed for Mac.
Be sure to open BG3 from the file in
~/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS/steamapps/content/app_1086940/depot_1419660/
Not through steam launcher.
