Add speed buttons to any HTML5 <video> element. Comes with a loader for YouTube and Vimeo
That's probably because this script doesn't support mobile websites. I have no plans to implement a touchscreen-compatible UI for this thing (nor is it even a realistic expectation, considering that google pushes the youtube app really hard on mobile so the number of people using youtube in a mobile browser would be very low, let alone a mobile browser that they've somehow installed a userscript extension on, which I didn't even know was possible), so you're kinda on your own with that one, man.
Doesnt work on m.YouTube.com