Is Hardware acceleration enabled in brave://settings/system ? Does restarting Brave, then opening the YT page and select between 720p/1080p to see if it helps lowering the resolution.
Actually everything was fine till few days back, i am using brave for over 6 months now. This issue started just this week, also i can now see ZOOM is also using a lot of CPU so it can be some other issue too maybe i should ask on Microsoft forum.