I went back and dug up this excellent post by Jimmy Chitwood in the 2018 thread. It discusses what the differences in 40 times really mean. It's applicable to every draft year.
"please pardon this double post, but many of you don't frequent the track forum, and i thought this post relevant to the upcoming NFL Draft ...
the above video (originally linked by RCSMAN) is highly illustrative and would serve as highly informative in "debates" regarding 40-yard dash times in football. if one watches closely, you will observe the VERY marginal separation between the winner, Lemaitre, and the third place finisher, Vicaut. their respective times over 60 meters "seem" large: 6.57 to 6.66, but in reality there is less than one full stride separating the two men.
this is significant, as it illustrates how truly small the difference is between a "blazingly fast" time of, say, 4.4 and a comparatively "slow-footed" 4.5 over 40 yards. in actuality, the separation between those two times is, at most, one stride. and on a football field where running in a straight line rarely happens, the difference between times of a tenth of a second (or even two tenths of a second) is almost totally insignificant.
the idea that a "speedster" who runs a 4.4 in shorts with no pads on is in a separate league from a "slug" who runs a 4.5 or 4.6 is, at best, misguided and, at worst, intentionally misleading." Jimmy Chitwood
edit: I don't know why the video didn't come through. If you want to see it go to the 2018 NFL draft thread and go to page 9, February 10.