To slow down your video, you have to use a multiplier greater than 1:
ffmpeg -i input.mkv -filter:v "setpts=2.0*PTS" output.mkv
For example, to double the speed of the video, you can use:
ffmpeg -i input.mkv -filter:v "setpts=0.5*PTS" output.mkv
If you have audio, see comment below about also slowing down the audio.
Thanks for catching this!