convert high -speed video to normal by subsampling frames
Solution 1:
If you want to keep real-time i.e. 1 second of live action is played out in 1 second of video then
ffmpeg -i input.mp4 -r 30 output.mp4
This will drop 3 out of every 4 frames.
If you want to preserve all frames but cycle through them slowly then
ffmpeg -i input.mp4 -vf setpts=4*PTS -r 30 output.mp4