With WRs, speed matters only as a cutoff, even size is overrated. Can't be so slow that the CB has time to recover.
Quickness, hands, body control, route running, RAC, all more important than straight line (40) speed.
With two deep common these days, how many WRs can run past people and locate the deep ball on a regular basis and win a jump ball? It's the guys who combine quickness and speed, and know how to lull a safety to sleep then turn on the after jets who get open deep (DeSean, Hill), not the guys who are fast with no deception.
QBs have to get rid of the ball under 3 seconds on most plays, in pads, given that you're not starting out of a track stance but standing up, I doubt the fastest WR reaches 20 yards out before the QB releases the ball. (deep balls require a leap of faith, throwing the ball before or just as the WR makes his move and letting him run under it),
So most passes tend to be in the 10-20 yard range and basically require the WR to either have a feel for the soft spot in a zone or being able to make that one well timed cut that creates separation.
Marvin Harrison had great speed, but what made him a great WR was running the entire route tree.
Isaac Bruce was timed at 4.6, but was an elite route runner.