I wonder if he is talking flat out speed, or quickness. Marvin ran a 4.54 40, and the fastest recorded by lasers ( since 1999) was a 4.24 (in 1999) and the most recent being a 4.28 in 2011. 4.4 to 4.5 seems to be about the norm and unless we have lasers in Iowa city, I don't think you could get an accurate enough time to say we are way slower. Infact, I think Randy Moss ran around a 4 to a 4.4 and he is pretty fast. All this assumes there was not a huge drop off after Marvin.
I think he is talking burst. Infact I would bet on it. So again I ask are we running more s&a drills this year? We really need to do so on both sides, for all these changes to have max sucess.
Let's look at the difference between Moss and Marvin to see what the difference might mean.
.14 seconds @ 18 mph is ~3.8 feet in a head to head sprint @ 40 yards.
Is that significant enough for good separation from a defender?
If all plays happened at 40 yards from the LS then yes.
Against a defense don't forget that the DBs start off backpeddling so the gap may be even larger unless a defender is faster in some manner.
Mitigating factors include reach, vertical jumping ability, soft hands, cutting ability, and instantaneous acceleration, what some (including me) call football speed and what I think you mean by "quickness", others "explosiveness" etc...
There are guys like Wegher, and hopefully Garmon, who seem to be able to get a step or two on a defender. Their terminal speed may be the same, but the 1 or 2 steps that overcome inertia lag seem to make the difference. If the defender has a higher terminal speed then he will eventually catch the receiver/carrier, but not before some positive yardage is gained.
Marvin has very high terminal speed because of his very long stride. When he catches the ball in the open field, he's gone. In coverage it's his other factors that make him a good receiver. Moss is more of a break away guy, he seems to be able to get a step or two on a defender and maintain that separation after catching the ball.
I think GD wants break away speed guys that have the ability to get past defenders quickly on the outside and then let the play develop after they have the ball.
That would be a paradigm shift for Iowa, where the model has been to let the play develop and then move the ball.