Like others have mentioned on here already, ProMotion isn’t working in a lot of Apple’s own apps. Hopefully this will be fixed soon.

I’ve found that when playing 1080p60 (or higher) on YouTube on Safari it’ll stutter. To “fix” this, change your Refresh Rate to 60 in the Settings.

EDIT:

Not sure if it was an Apple fix or YouTube fix, but all video plays buttery smooth now.