First off, from the video posted above, you don't even see the start of that play. What type of cushion was Talib giving? Do you realize Talib isn't exactly a burner himself? Smoking a 4.5 CB that may have been pressing doesn't exactly mean that you're quick. It certainly doesn't mean he runs a 4.3.
How anyone could look at those videos and think he's as quick as Welker is beyond me. Most scouts have pointed out that he's probably going to have separation problems in the NFL. Many have speculated that he'll run in the 4.5-4.6 range. He's not a slot guy. He's a fine player, but he's more of a poor man's Keyshawn Johnson IMO.
Edit: After going back and looking at another angle of the Talib smoke out, it turns out Talib was pressing Nelson, and that Nelson didn't get by him until nearly twenty yards down field...where he then proceeded to pull away.
Like I said, he's a long strider with good top end. He's just not overly quick.
http://youtube.com/watch?v=ek0ANQ8bRfo