Use the velocity envelope. It will be a green line. Take it all the way to the top.
That isn't going to get you to 15x the speed, so what you'll need to do is render out a new file at that speed, bring it into the project, and do the same thing over again until you get the speed you want.
Two things to remember:
When you increase the velocity, you won't decrease the length of the file on the timeline. You'll need to do that yourself by dragging the end of the clip.
I've never tried to do this, but I suspect you'll get some image quality loss over the multiple speed increases and renders.
Alternatively, go in and manually eliminate everything except every 15th frame.
'nother option is to Ctrl+left click and drag at either the start or end of your clip inward. You'll reduce the size but as mentioned before, you may need to do this a few times to get it down to a min.