Going FAST doesn't really cost much in electric car, unlike gas engine having to put in more cylinders and stronger transmissions, electric car's motor doesn't need to get much bigger to be powerful for the same price (maybe another 50lb for better insulation to double the voltage? I don't know), and the power electronics also don't weight much even if you double them. The biggest weight gain is the weight of the battery pack which doesn't change much when you want to double the power and reduce the range.
What this means, is like these days we have gasoline cars starting at minimum 110 hp instead of 60 hp, because it doesn't help going to a smaller hp engine in cost, emission, or mpg, EV wouldn't need to go much slower anymore as torque in EV is cheap if not free.
The only reason I see going "slow" make sense is to reduce suspension weight and crash safety, savings in tire and cooling system, etc. Those might save money if you can make the car slower and reduce the performance there.