While on the topic of ffmpeg uses, here is a script I posted that speedup videos by a factor of 1.5: https://news.ycombinator.com/item?id=22584131 (use case record an unscripted screencast or demo with lots of pauses and uhms, then speed it up to make it look like you are super well prepared and caffeinated --- hidden bonus: if demoing a software product it will appear to be 33% faster!)