Well Youtube performance used to suck under Firefox as well to the point that I felt that running youtube without the addon "Youtube Classic" in firefox was unbearable.
Playback or rendering ? I know my browser has performance issues because it doesn't have hardware-accelerated playback for VP9, I don't know if FF is in the same basket (sounds like it might be). Having an addon that only enables h264 solves that.
YouTube started using Shadow DOM V0 (which is deprecated) some time ago, which is implemented only in Chrome/Chromium, as a result all other browsers render it slower.
Not to mention pulling the Windows Phone Youtube app that Microsoft wrote because it wasnt made with html5. Nevermind that the android or iphone app didnt support it either.