You can change the playback speed of video you are watching on YouTube. You can watch videos in slow motion for watching its details or switch its fast motion mode etc by clicking the ‘Settings’ icon present with bottom right buttons on YouTube video player then select the speed from ‘Speed’ selection dropdown box.
If YouTube Video Player Is Not Showing Playback Speed Controls
If in-case your YouTube video player is not showing video playback speed settings option then you must be using default Flash based YouTube video player. So visit www.youtube.com/html5 and request your HTML5 player:
OR, Alternatively you can use VLC Player for playing any YouTube, Vimeo or any other online videos and VLCPlayer already provides you various video controls including playback speed.