The best exercise you can do for speed is squats. Everything you do to be fast on the field can be translated into a squat.
1) It builds your legs. Your legs need to be explosive in order for you to be fast. You don’t see too many guys with skinny legs running a 4.4 forty. You need to squat heavy too. Squatting heavy activates your fast twitch muscle fibers which are the same kind of fibers you use when you run.
2) The bottom of a squat is a position of no momentum. If you can be explosive from that point, you will be explosive on the field. For example, a WR is standing at the line and he has no momentum to propel himself into the route. The Canadian league is a different story however. But that WR needs to generate explosion from a point of no momentum To get into the route. And that is the same for any position in certain situations.
Squats are the BEST exercise for any athlete trying to get faster.