There's absolutely a reason he's not running the 40. He's probably a 4.6 guy (Mark Ingram speed). Which IMO does not matter and RB speed is pretty overrated but he's definitely not what I would consider "fast" for a RB.
The list of pro bowl level RBs under 5'9 with a 4.55-4.6 40 is not that long. I'd imagine it's maybe a few. Which again is fine because he's such an anomaly.