Faster Video Speed

ffmpeg -i input.mp4 -filter:v "setpts=0.5*PTS" output.mp4

Slower Video Speed

ffmpeg -i input.mp4 -filter:v "setpts=2*PTS" output.mp4

The lower the PTS value, the faster the time-lapse video is generated.  If you use a larger value, the video will display in slower motion.
Adjust video and audio

If you care to sync the audio speed with the video speed, things get a bit more complicated:

ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4

More confusing is that the atempo setting seems to require reverse logic to setpts ; i.e. doubling speed seems to require a different multiple.

I recently used this slow-mo technique on a highlights video downloaded from YouTube to draw my own conclusions about a specific play.  The ability to manipulate video so easily, without a UI, also makes for easy automation of media, especially when you combine this technique with clipping video -- you get just the frames you want at the speed you want.  ffmpeg FTW!