Downhill horses can sprint, which is exactly what cow horses have to do. They need to go from zero to a milion miles an hour, even if only for the second it takes to stop a cow from going the wrong way. Thoroughbreds are also built downhill, though not as much as quarter horses, and quarter horses are faster over a quarter-mile than thoroughbreds. Presumably if being downhill impeded speed, it would have been bred out of thoroughbreds and racing quarter horses, as speed counts for more than anything when it comes to racing (well, yeah).
Well-trained, downhill horses with good conformation (and yes, that includes being downhill) know how to get their hocks beneath them to lift their shoulders up to move a cow, as a good uphill dressage horse does, though for different ends.
I don't know the mechanics of why being downhill makes them able to sprint the way they do, but it seems to count. Like anything, of course, too much downhill is not a good thing. Much in the way a short back is better than a long back, provided it's not too short. But downhill seems to count. A lot.