If the engine was running flat out, for max blade speed, and the blades were very sharp, I can see how in certain circumstances that mowing at 10mph is possible.
If you don't miss that bump or hole at 10mph, not only does it shake up the operator but it adds wear to the machine, some would say abusive wear. We don't drive our cars or trucks at full speed do we?
I think every property is different and one has to learn one's lawn and where the "sweet spots" are where you can fly.
I have a couple of those spots, where I can tromp on the JD's treadle and go max speed over level ground while cutting about an inch of grass. But mostly I'm going about half the potential speed because with a lawn full of trees and shrubs, that's what is needed. Most of us don't have radar guns, but I bet the average mower here doesn't exceed 5mph even with ZT mowers, out front mowers, etc. Particularly if you are using cruise control, you don't want to set it at a speed you constantly have to disengage it so average set speed comes down a little.
Commercial mowers have to maximize efficiency but most of us don't. We want a blend of affordability, reliability, performance and comfort. Take away any one of them and things fall apart.