When watching videos on Ubuntu 15.04, youtube videos are being EXTREMELY choppy. HTML5 player is on, and it's still choppy. I turned off hardware acceleration, and same result. It's getting to the point where it's annoying.
You could try 1. Alter the settings on you-tube to a lower quality picture. Its unlikely you would notice the difference in quality but it makes everything faster.
2. Clear the browser cache
3. When the video starts, pause it for a short while to allow more to be cached before restarting and watching it.
4. Find a download utility and media player, download the full video and watch it on the media player. This is actually less hassle than it sounds.