The only way a backpack would make my ride any faster would be by making me so uncomfortable that I'd be racing to the end so I could take the d*** thing off. If you're really concerned about the aerodynamics of a pannier then try a saddlebag instead. I used the Carradice Camper model for commuting for some time. Lots of room, doesn't affect steering like a handlebar bag, and is directly behind the rider in his slipstream. But I never noticed any speed or effort difference between that and carrying the same items in a pannier.