I'm not buying fastest if you're talking straight line speed. Mike Wallace would beat him in a 40, as would some others. ETA: And of course Holliday would beat him in a sprint too.
But he has the shiftiness, and he has one of the best combinations of being able to make moves and maintain top speed. On the 3rd TD run he had of the day, the last one of the 2nd half, two UT players had him penned in at the sideline. At full speed he changed direction and split both defenders, and they barely got a hand on him. It looked like he never broke stride. That's what he does that is more exceptional than most players. It's his ability to make moves at full speed, and it often doesn't even look like he's making a cut.