This has been posted here, on the Steam forums and all over Reddit. The update on 3/7/24 has made gameplay under MacOS impossible.

Since first installing, I have been getting around 30+ fps at 4k, ultra gfx settings. Haven't played the last 6 days, now suddenly getting 5fps. Even at 1920x1080 it's only around 10fps,Mar 8. Feels like hotfix #21 may have broken or changed something for us mac folk? I've tried reinstalling, clearing caches, etc. no luck. Plus as I've said, many other mac users are reporting the same.

Hopefully a fix soon?