The pair was only vulnerable not because of lack of pace, but because of Rafa's self-initiated blunders(like the one against Drogba at CN). It was pep's preferred CB pairing during the sec. half of 2008-09, and he would've most likely continued to play them if not for the Mexican's injury against Chelsea. Rafa was slow, but an excellent reader of the game, likewise Hummels, except the german is a more well-rounded defender. What Rafa and piqué shared was a working chemistry and understanding/reading each other's movement, and its all that comes down to.
The pace issue has been overstated on this thread, and elsewhere. The next poster who comes here whinging about it, will get it from me.