What you need is game speed, as they call it. Remember usain Bolt? Slow out of the blocks - I don't know what his 40 speed would be, but when he hit his stride and gor it all moving, he was fast - real fast. That's an extreme example. Once off the line and moving, we can all name a lot of receivers who played faster than their 40 time.
On the flip side of that coin, Greg Gaines is there within a step-and-a-half. He catches QB's and RB's with a quick burst that seems to come out of nowhere although his 40 time is probably slowerm than theirs.
Larry Fitzgerald didn't have a fast 40, but could his another gear once he got rolling and was seldom caught (I didn't say never - just seldom) from behind.
Warner used both Fitz and Anquan Boldin deep. They were both physical, had hands (Boldin's were actually better, i thought,) and Boldin, especially, could go get and hang on to contested catches.
Our young WR from Montana (I root for my MT kids, Hardy and McCutcheon) reminds me of Boldin - taller and skinnier, not as heavy, but plays with that "My ball - I'm getting it and keeping it!" attitude that Boldin carried, and he does it in much the same manner. Is he ready, yet? I'd love to see him but I'm trusting the coaches to evaluate how well he can get off the line against NFL corners, how well he runs routes and adjusts post-snap, and to give him time to develop timing and awareness with Stafford. He's another with "slow" 40 time, but who comes ready to play football.