Worth noting: A Pi 1 will decode 1080p h264 video flawlessly. I used one as a media center for a long time, running OpenELEC (now LibreELEC). Moving around the UI was a bit sluggish, but video playback was smooth.

YouTube is an absolute clown show. It's so bad that I'm certain Google devs are actively making it terrible on purpose. I use Newpipe on an older (but not that old) tablet. Whenever Google breaks Newpipe and I have to use a browser, it takes like 30 seconds just to load the page.

Decoding video is trivial when you have hardware decoders.