Speed up by a 2x factor :

ffmpeg -i input.mkv -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mkv

Or a x factor :

ffmpeg -i input.mkv -filter_complex "[0:v]setpts=<1/x>*PTS[v];[0:a]atempo=<x>[a]" -map "[v]" -map "[a]" output.mkv