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.
I 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.