[FFmpeg-user] hflip so slooooooooow

[JD]

Hi all, should horizontal flipping be as slow as this:
frame=27067 fps= 12 q=28.0 size=  143993kB time=00:18:06.27
bitrate=1085.9kbits/s dup=0 drop=35 speed=0.47x

the command was:
$ ~/bin/ffmpeg.d/ffmpeg -i video_mnt13.mp4  -vf "hflip" -y
video_mnt13-hflip-m1B6z-54q_0.mp4
ffmpeg version 3.2.4-static http://johnvansickle.com/ffmpeg/  Copyright (c)
2000-2017 the FFmpeg developers   built with gcc 5.4.1 (Debian 5.4.1-5)
20170205

how can I speed it up? (I thought that flipping should not need
transcoding)???