The OP should ask
ECV manufacturers for the answer to the original question. People at Disney rent what's available, and have no say in the ECV's features. I understand that Disney-furnished ECVs reach a slower max speed.
There is a weight range for people operating an ECV, and those of higher weight need more power to push forward. More power is also needed for inclines. A battery that is weak from age or lack or charge requires more power. Combine incline and heavier weight, and a high amount of power is called for.
One could look at the ECV panel and see the Turtle-to-Hare control as a speed control. But it is also a power control for factors listed above.
While we're at it, why do Segways go so freakin' fast? Woe be the day guests have them at WDW.