Well, just out of curiosity, I did the math. The rotor tip on a 400 ft. diameter windmill (about average up here) at optimum wind speeds producing 15 RPM (goes around once every four seconds) is going over 214 miles per hour.
Fifteen RPM is the low end, 20 is the high end, and produces rotor tip speeds in excess of 285 miles per hour.
(>splat<)