Here's how. Download VLC and run it. Under the File menu, "Open Network ..." and paste in the YouTube URL. Then, under Playback, select Faster or Slower as needed.
Unfortunately, there's a catch: YouTube throttles download speeds. You can only download one second of video per second. So you'll have to wait for the video to finish downloading and then watch it. (Hey, at least you don't have to sit there and watch it download!)
Alternately, you can use VLC's Streaming/Exporting wizard to save the video to disk first.