ffmpeg -i input.mkv "setpts=0.5*PTS" output.mkv // 2xspeed

ffmpeg -i input.mkv -r 16 "setpts=0.25*PTS" output.mkv //4xspeed without at 16fps

ffmpeg -i input.mkv "setpts=2.0*PTS" output.mkv //0.5speed slowmotion