Why it matters: For wide receivers and running backs, breakaway speed can be the difference between a modest gain and a game-changing play. A measurable combination that NFL evaluators pay close attention to for cornerbacks is length (height and arms) and 40 speed. Safety is another position to keep an eye on, particularly for players who will be asked to cover a lot of ground in the deep middle of the field. The chart below shows the most desirable times, the average combine times over the past five years and the times that should raise a red flag for evaluators.

Past standout: Saints WR Brandin Cooks. This is a good example of a player who looked fast on tape coming out of Oregon State and showed off his speed in the 40. He ran a 4.33 prior to being drafted in the 2014 first round. That is tied for the fourth-fastest WR time of the past five combines. Cooks' speed has certainly transitioned to the NFL. He tied for the league lead last season with six catches of more than 40 yards.

Impact of WR Mike Williams not running the 40

Mike Williams not running the 40 a 'red flag'

The NFL Insiders questions WR Mike Williams' decision not to run 40-yard dash because it won't alleviate concerns about his top level speed.