Fast enough for what? Short, sharp dashes after rabbits - I very much doubt it. Long runs with no cover in sight - possibly. The way I'd look at it is to ask if a straight deerhound would be fast enough. If the answer's yes then crossing to the bull might possibly still be, but I think it highly unlikely that the pups will be faster than the deerhound. It's not a cross that I'd want personally, but each to their own...