First time you change the speed of a video you need to use the mouse (left clicks inside the control).
Control is visible at the top left of current video.
Later you could use the shortcuts.
Its remembers speed for nearly all following videos, all playlists or so. Enjoy
if its not clicked the google default novigation shortcuts are active.
for example:
Numbers 1-9 - Go forward a percentage (1 goes forward 10%, 2 goes forward 20%, etc.).
Right arrow - Go forward 5 seconds.
More details see here https://sites.google.com/a/umich.edu/going-google/accessibility/google-keyboard-shortcuts---youtube
Its tested with Google Chrome Version 42.0.2311.90
Reviews of HTML5 Video Speed permanent Control extension