Just a shot in the dark here, but do you have ambient mode enabled? (that annoying glow light around the video). I’ve read from various people that it can use an insane amount of cpu on some devices. Disabling it fixed the performance issues they faced.

The first time it paused my YouTube video inside Firefox when a call came in on my phone, it blew my mind. And then when the call ended, it just casually resumed the video. I love it. That, the clipboard syncing and easy sharing of files between devices make it absolutely awesome to have.