I did as you suggested, I parked up 3 meters away from a brick wall. I turned on the ignition again and let it go into recording mode.
After 30 seconds, I reversed back with a hard lock on and parked up so I could check the video quality on the left & right edges whilst in motion.
On reviewing the video I noted that whilst stationary no part of the image was blurred. When reversing I noted that some of the frames were blurred, but some clear.
I used VLC in full screen mode to view the video so I could check it frame by frame in its worst mode, then, when viewed @ 50% size it was very acceptable.
So your diagnosis of motion blur must be correct, I bow to your superior knowledge DT MI.