You will need to install a user script manager extension to install this script.
A simple script to change the playback speed of the video by using the + and - keys on the numpad. Pressing + or - sends a keypress that increases or decreases the speed (shift + , or . see YouTube-shortcuts).
You can also reset the playbackrate back to normal by pressing the * key.
This is especially useful for azerty keyboard users, because there doesn't seem to be a way to increase the speed using the already existing shortcuts.