Ground speed makes almost no difference to the rate of destruction on a discbine. Rocks and rough ground do, in that order. I've got one of those here who likes to preach about how the world needs to run at 3 mph because that's how it was always done and it saves on equipment... then drags the rocks 100 feet down the field because there isn't the presence of mind to stop and lift the mower. Then comes home needing 1000 bucks worth of parts. The arabs love him too since it doubles the fuel bill to do the same amount of work.
The way I see it... if you want to drive at 3mph you might just as well stay with the haybine and run it on a smaller tractor and save the fuel.