I've been using Premiere Pro happily, no problems with playback.
I just upgraded the operating system to Big Sur, and now the playback is lagging horribly - making it much harder to edit. I've tried rendering clips, tried adjusting resultion - in other words, all the generic things you do to try to resolve playback issues, but those have not made any difference.
My machine is pretty respectably specced. If what you're suggesting is that there is no solution to this short of downgrading the entire operating system, then I suppose that's probably the prompt for me to switch to Final Cut Pro. Disappointing, but it's unusable the way it is now.
Have you tried normal troubleshooting procedures, like, trashing preferences and deleting media cache? Is your Documents > Adobe folder have R/W permissions set properly? The Premiere Pro app have R/W permissions? Sometimes permissions can go amiss on macOS updates. Check these things and report back. Do also inform us about your media. H.264 media is non-performant no matter which system you are working with, if that is the case.