Video playback speed was mentioned on the useful habits repository thread a few weeks ago and I asked how I could do the same. Youtube's playback speed option is not available on all videos. Macs apparently have a plug-in you can download, I don't own a mac so that's not helpful. You could download the video then play it back, but that wastes time. I just learned a solution that works across all OS' with out the need to download the video first.

copy youtube url, ctrl v on vlc mainscreen