Floyd is fast (not the fastest) but his reflexes are more impressive. Some will argue he is probably as or if not faster then pac, but I think it has more to do with him having quick reflexes that allow him to get in a better position to strike.
Prime Oscar and Mosley were easily on par if not faster than Floyd. Prime Terry Norris, Ike Williams, Willie Pep, Hearns, Benitez, SRL, Montreal Duran, prime Toney, Tyson, Judah, Camacho, Khan, Taylor, Rj, and Pacman. It's really Floyd's timing that is most remarkable, moreso than his speed.
I think its the combination of speed, timing, and reflexes. I dont think one is more important than the other when it comes to Mayweather's sucess. Pacquiao's speed for example is comparable to Mayweather's, but he's far from Mayweather's ability in timing and anticipating, as well he's nowhere near Mayweather's league when it comes to reflexes.