To increase speed of a video by a factor of 1.5x:
ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=PTS/1.5[v];[0:a]atempo=1.5[a]" -map "[v]" -map "[a]" output.mp4
We can use this to adjust the speed of all the videos in a directory (Linux and macOS only):
mkdir -p output