If I'm walking alone I do about a 10 minute mile but I think I walk fairly fast.
You are either measuring incorrectly, or you are fully aware that the vast majority of the population walk nowhere near that fast, as I'm sure you've noticed as you blow by them.
6 mph. First search suggests Olympic race walkers average about 7.7 mph. Another site suggests power walkers max out at 5.7 mph.