Let's be very clear about one thing. You can run a 4.2 in pro day, but run a 5.4 in pads. Lamb is way faster on the field. Simply having a good 40 yrd dash doesn't make you an explosive receiver. It's just a piece of measure to talent.
Lamb has freakish athletic ability outside of speed (and by no means is a 4.5 slow when you're not even a track guy). He has great feet, space awareness, and above average route skills/hands. Gallup is usally the guy that takes the top off of defenses when healthy, but he's not fast right?
The difference between a 4.4 and 4.5 on the field isn't that big. Unless you're running a 40. Then it comes down to who comes off the line faster and so on.
In terms of on the field, if a receiver can punch you in the chest with two hands, then juke you, and you don't measure up in lateral, and natural agility then all of that straight line speed means nothing. You're burnt.
Look at Micah. Total freak. Can be in the middle of rushing the line, turn on a dime and chase a guy 20 yards down field with a 5-8 yd head start and STILL catch up to bat the ball down.
Those kinda guys don't grow on trees. It's just not that simple. Speed is just one important part of the position.