I commonly watch dev talks and walkthroughs on youtube at greater than 2x speed because the talkers make an effort to talk slowly for non native English speakers.

Just pop open the console with F12, enter the following and hit enter:

$('video').playbackRate = 2.5

Change `2.5` to whatever number you wish.

Press up in the console to bring back your last command faster with the cursor at the end so you just delete the number, type a new number and hit enter.