Evidently you can teach speed.
"First, it's about mechanics. Fix a player's fundamentals and he'll naturally get faster, Kingdom said, especially if he's either on the bottom or tapped out his genetic makeup. And Kingdom didn't just talk about shaving a few milliseconds off -- he said a player could go from a 4.5-second 40-yard dash to low 4.4 or even high 4.3-seconds."
http://espn.go.com/blog/arizona-cardinals/post/_/id/5565/gold-medalist-kingdom-can-teach-speed
"First, it's about mechanics. Fix a player's fundamentals and he'll naturally get faster, Kingdom said, especially if he's either on the bottom or tapped out his genetic makeup. And Kingdom didn't just talk about shaving a few milliseconds off -- he said a player could go from a 4.5-second 40-yard dash to low 4.4 or even high 4.3-seconds."
http://espn.go.com/blog/arizona-cardinals/post/_/id/5565/gold-medalist-kingdom-can-teach-speed