A good 40-yard dash time at the NFL Scouting Combine only guarantees one thing: that a player is fast.
It doesn't automatically dictate that a specific draft prospect will transition to pro football and immediately dominate as a rookie.
A long and prosperous career does not instantaneously evolve after a player hits among the best times at his position, or even throughout all combine participants. The upside to the 40-yard dash, however, is that it will only help increase a prospect's stock.
Take Tyrann Mathieu as a prime example from the 2013 combine.
The guy is clearly a great athlete and possesses the physical capabilities to last in the NFL. But there are character concerns that unfolded during his days playing for LSU. So, whether it's off-the-field issues or sketchy production, risky prospects will always exist during the draft process.
Note: Full view of official 40 times from the NFL combine.