How Absurd Is A 40 Yard Dash Time?

To answer the question, it's not the least bit "absurd." It's a perfectly valid measure and point of comparison. Just like all the others.

As has been pointed out several different ways, .2 seconds IS significant with all else being equal. On a 10-yard slant, it's .4 yards, or just over a foot. Which can easily be the difference between breaking up the play or allowing a completion for a first down. Over 20 yards, it could be the difference between breaking up the play and missing altogether, allowing a HUGE play after the catch.

Sure, a lack of speed can be compensated for. But compensating for a weakness has its price too. Larger cushion = vulnerability underneath. Etc. etc. etc. Anyway you slice it, speed is an asset and lack thereof is a liability, all else remaining equal, which is how you have to make these comparisons.