In the past I have seen 1.5X/2.0X playback in YouTube working on some YouTube videos. To ensure this was not due to old YouTube videos, I chose a random 2014 Superbowl commercial. I cannot see any speedup playback controls.https://www.youtube.com/user/mmschocolate?v=q3Z4TQKGXbEI am using Chrome on OS X Mavericks.Update  After a half dozen attempts the speed setting is now showing up - on the same video it did previously. I have no clue what has caused the speedup capability to reappear: I did not make any Settings changes in the interim.
When/how is 1.5X/2.0X speedup playback available in YouTube with HTML5 browser enabled?
youtube;video
Google youtube html5 - this will take you to a page that allows you to select html5 as the preferred player.  HTML 5 = player - you have the speed option.  Flash = player - no speed option.