I will admit that i never watched him in college. I only have watched all the youtube highlights and draft show highlights of him.
But with that said I never thought he looked that fast in college highlights. I never really saw him as a flat out burner. What I saw was a guy with amazing hands and body control.
The NFL is so fast that very few players will stand out for their speed. Every corner and safety in the league can run 4.3-4.4 40's. So in order to look fast you need to be like Chris Johnson and be able to run a 4.1-4.2.
I think what will make Dez is great player in the league will be his ball skills. He has the abilty to go up and get every ball that comes his way. His hands look huge and strong. His speed is enough to make the defense respect it. But not world class. His size is nice. He seems hard to take down after the catch also. If you look at players like Larry Fitzgerald or Andre Johnson those guys are what Dez should be able to become. If you are expecting Steve Smith or Desean Jackson type of player than you will be disappointed.