They used to also use the unreleased version 0 of shadow DOM for building the Polymer UI, which - being a Chrome-only prototype - understandably didn’t work on Firefox, and therefore instead used a really slow Javascript polyfill to render its UI.
I haven’t checked on it lately, but I imagine they must’ve changed at least that by now.
I’m pretty sure someone discovered that is true recently, but can’t be assed to try to find it right now.
They used to also use the unreleased version 0 of shadow DOM for building the Polymer UI, which - being a Chrome-only prototype - understandably didn’t work on Firefox, and therefore instead used a really slow Javascript polyfill to render its UI.
I haven’t checked on it lately, but I imagine they must’ve changed at least that by now.
Here you go:
https://www.reddit.com/r/youtube/comments/17z8hsz/youtube_has_started_to_artificially_slow_down/