I’ve noticed this on two different builds across two different machine when I was using Firefox to watch youtube videos and using HTML5. The videos used to take forever to start up whereas on flash they would start up instantaneously.

It’s strange but I’ve only noticed this issue on the Gnome based builds of Antergos. I did not have this issue in XCFE. Not really sure what that’s about.

Although I used to solve the isse by forcing youtube to play flash, the playback speed control has recently become an essential function for me and it’s only available on HTML5. So, I looked for a solution. There’s a lot of threads about this issue but this is the only solution that worked for me, so I thought I’d share it with you guys.

source: http://askubuntu.com/questions/761296/laggy-youtube-performance

go to about:config and set layers.acceleration.force-enabled to true. It doesn’t work a 100% of the time but the lag problem has diminished drastically.