Here is a quick example on how you can use FFmpeg to slow down a video.
Slowed down video by 50%:
ffmpeg -i input.mp4 -vf setpts=2*PTS -filter:a atempo=0.5 output.mp4
-vf setpts=2*PTS
will slow down the video twice it’s original speed.-filter:a atempo=0.5
slows down the audio to 50%.
Unfortunately, the caveat here is that the tempo must be in the range of 0.5 to 2.0.
Also, you do not get that slow-motion down pitched sound as many slow-motion videos have.
Sources: