Eifert was "very impressive" with a 4.68...
Teo "slipped drastically" with a 4.82...
Assuming these numbers are what they would actually run in pads, with a ball, and for some reason they were in a straight line sprint, the difference in ground covered in 5 seconds is less than 4 feet. Now if that was the case a good quarterback would have a big window to throw into (also taking into account Eifert's height/length).
But now we are on a football field and the many of the players that Manti would be covering in a man scheme are not going to be running 4.68 or better... and then you add the fact that the player is not likely to be running in a straight line (and presumably neither is Manti)... and then you add to that the fact Manti would presumably not be in press coverage so would likely have some cushion for the receiver... how much of that window is left?
Can he man up on Vernon Davis? (Who ran about 4.4 at the combine...) Doubtful. But not many people can. But can he stick with the majority of people he would be assigned to cover in man scheme for about 5 seconds, it would seem pretty likely...
Finally, I think the "lack of top end speed" speaks to what is probably his best skill which is being able to read and diagnose plays... I don't think he "lost speed" over the course of the year, but somehow he was able to make plays all year long. Even during the Alabama game, which was not his best game, he seemed to diagnose plays faster than most people on the field.
