I have been paying a lot of attention to the success (or, in many cases, failure) of big possession receivers since the 1995 draft. As a Rams fan whose teams had been torched by Rice and Taylor for years, I cringed at the thought of the 49ers taking JJ Stokes. Stokes was big and strong, impossible to bump, and had fantastic hands. The only knock on him, they said, was his lack of speed. While the other two top prospects that year (Joey Galloway and Michael Westbrook) had both turned in 4.4 or better times, Stokes had run a pedestrian 4.65 at the combine. The 49ers dismissed it as unimportant; after all, West Coast Offenses across the league were thriving with big possession receivers lacking speed. Cris Carter, Herman Moore, Alvin Harper...hell, even the greatest, Jerry Rice, was not a burner.
While Stokes would go on to have a decent career, he never scared anyone, and his lack of speed was always attached to his lack of production. The game steadily evolved out of the methodical WCO and speed and quickness became more and more important. "Explosion" is the thing teams were starting to look for, and the big slow guys were having less and less success as time went on.
Dwayne Jarrett was the last guy who fit into this category that was really even considered by NFL scouts as a high-round pick. There were two sides of the argument; on one side, his size, strength, hands and route-running ability would be enough to have continued success despite his lack of speed and explosion. On the other side, his lack of speed and explosion would prove to be too much to overcome in the pro game; CB's would be able to sit on the out route, he wouldn't be able to get separation, and he would never require a double-team. In the end, the latter proved to be the correct analysis.
I haven't done a study in a few years, but for several years from about 2003-2007 I documented the average forty times from the Combine. WR's and CB's at that time were averaging around 4.45-4.48 depending on the year. So when you get a guy who is running a 4.6 or above, it really doesn't matter how you want to cut it...he is slow and slow doesn't win in the NFL anymore.
So my take on it is this...if Alshon Jeffery runs in the 4.6's, I don't care how good he is at everything else. He is too slow be a game-changing WR in the NFL, and I don't believe he will be anything special.