On some brands you use a speed control lever to control the max speed setting and pistol grips or similar system to slow it down from that setting.
Toro uses a push to go t bar with no seperate speed system.
Hustler uses a H bar twisting system like a motorcyle to controlspeed.
Gravely uses a system that reminds me of ZTR speed control handles, just push to go, pull to reverse. I know they can go 8mph. Much faster and you may have a control problem on rough ground.
I think the Hustler super walkbehinds are the fastest 8-10mph
and the large tires make the ride smooth enough to use more of the speed and the extra hp helps also. the controls of the hustler don't have a max setting (YET!) I think hustler is working on something to set the max speed. for now you just turn the handle forward as fast as you want to go and turn it back to slow down and reverse. IMO and some others it is the easiest system to learn and use. try one and see for your self if you like it.