Here is the original video:
Slowed down 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 “slowmo” down pitched sound as many slow motion videos have.