There are speed controls that show up with the player controls when playing a video. You can locate it at the bottom of the video screen. The default setting starts at "1x" and cycle through the different speed options when you click on the icon.
Alternatively, if you want finer or wider control over the playback speed, then you can use a Chrome extension like Video Speed Controller.
Along the bottom control bar you will also see controls to allow you to skip forward or Backward by 15 seconds. This is a useful tool to allow you to repeat a concept it you want to hear it again to clarify your understanding.