have been using bazzite for a while now and don't have issues with Firefox, could be tests Google are running. they recently are targeting users of adblockers and artificially setting high load times in hopes users disable them. a decent alternative is freetube (on flathub) which is anonymous so you can't login but you can import playlists and includes ad+sponsor block. they're pretty fast at fixing Google's attempts to break it too, sometimes you might have to change instances to get it working.