5.15 + kms + xorg + kwin compositor is a very broken combination (won’t boot). Disable the kwin compositor, then it runs fine.
Recently, kms + wayland + plasma has improved. But Firefox + youtube will freeze the desktop. If I recall correctly, Sway ran fine with kms.
I believe picom is used as the compositor for the RPiOS desktop. If your DE allows running an alternate compositor, you might give it a try.
Edit: For the firefox + youtube, maybe it is because I did not setup Firefox for Wayland. I will try to confirm this later this evening.