Except that if a dishwasher was significantly faster, people would love it. 20 minutes would be enough to make people go "wow, that's really nice", and that would become the new "cool" dishwasher.
There are plenty of dishwashers faster than 20 minutes, they're all in restaurant kitchens which do have a speed requirement. Several bleeding edge people have installed restaurant style dishwashers in their private homes but the requirements (in terms of energy use, danger of the chemicals involved, noise, temperatures reached) make them not a good fit for almost all homes.
even image one that does not require water, electricity or chemicals! ;)