#Youtube intentionally slowdown your loading time if you use #Firefox.

Video demostration: https://www.reddit.com/r/youtube/comments/17z8hsz/youtube_has_started_to_artificially_slow_down/

(Possible) fix:
1. Click on uBlock icon
2. Dashboard button
3. Add "www.youtube.com##+js(nano-stb, resolve(1), *, 0.001)"
4. Apply changes

Alternatively you can change your #UserAgent to #chrome.

Nice behaviour, #Google!