Note: You must opt-in for html5 for Youtube to be able to change playback speed
You've probably noticed some of the shortcuts and "easter eggs" in the youtube video player.
For example. You can use the ‘0’ key, while a video is playing, to move the play head to the beginning of a video. Press the left or right arrow keys will let you go back or forward in a video by 10%.
Thats also almost how you activate the hidden snake game, so don't be surprised. Or maybe you've already come across that.
Also, you may use your number keys to jump through sized chunks of the video – the 1 Key will move you to the 10% mark, 2 Key to the 20% mark, and so on.
Google (the owner of Youtube) has released some features slowly and sometimes hidden from the (average) user.
The Slow Motion or Speed up effects for example, don't work in most videos yet.
Sometimes you can go slow motion on some videos - look for videos that have the little menu like in the screenshot above.
You can then either increase the video playback speed by 2x, or slow it down to a quarter of the original speed!