Well, that was a constructive first answer...
From my modest experience, sure someone would have better words than me to describe it, but do you feel the difference when you put your hand out the window of your car at 20 mph or 100 mph? Same goes for reversers. The basic concept is that drag will increase at the square of speed, so if you use the reversers say immediately at touch down at 140 kts, they will be 4 times as effective than at 70 kts. Of course this is over simplified, does not take into account ram effect, engine approach idle speed, etc.
Corrections and comments welcome,
Hope this helps.
Flex