After years with "Enhancer for Youtube" I wanted to try something else, to eventually improve youtube's reactivity while using addons and this one definitely do improve youtube's reactivity (at least on my little computer).
Also this addon has an option to fix the video size (in pixels) which is great to avoid useless graphic computing (no scaling) and blur.

Many options and good UI.

I would add an option to hide youtube's sidebar completely (no space used).