You start each play not moving though so I don't think a moving 40 time is all that important. The NFL guys wants to see how well you can get off the line and get to top end speed.
90% of the routes you run, you're not going to run a straight line off the gun. You build up speed, change direction, and then go balls out - so you already have a portion of your speed achieved before your full on sprint. So that means you rarely attempt to get to top end speed right of the line of scrimmage. The true test of speed is how fast you get there, out of a cut. That's a combination of agility, balance and track speed.
So Allen Robinson can jump