Bug is reported here https://github.com/WebKit/WebKit/pull/806#issuecomment-1170759817. Animation is very janky on a MacBook Pro with M1 Max and ProMotion. The animation becomes even more janky when ProMotion is turned off and the refresh rate is set to 60hz fixed. Issue doesn't occur on WebKit 614.1.19 on a M1 MacBook Pro (no ProMotion, fixed 60hz refresh rate), so I believe this has something to do with the variable refresh rate.
*** This bug has been marked as a duplicate of bug 242183 ***
(not entirely sure why these duplicates keep happening but thank you!)