I really don't know but I have both a tb and an arab, and there both really fast. the arab is a barrel racer and the tb was never raced but her great grand sire was northern dancer. It would be cool to flat race them against each other and see who was faster but I agree with what one of ya'll said:
If you put a Quarter Horse, a Thoroughbred and an Arabian into the same race, the Quarter Horse would be the quickest to the quarter of a mile mark, the Thoroughbred would win the race but the Arabian would keep on going, and going and going!