I found a solution to have a high res and good frame rate with motion !!!!!!!
For that simply install mjpeg-streamer (that will give you good res + great frame rate) and tell to motion to use it instead of using /dev/video
More info here
http://www.lavrsen.dk/foswiki/bin/view/ ... pgStreamer
I am able to detect motion on video of 1280x720 at 15 fps.
Hope it helps