I mean without running a 40, anyone that has watched Jeanty knows he’s not “on the slower side”. The guy is undoubtedly explosive and clocked as fast as 21.7 mph this past season. There’s also plenty of very good RB’s 5’9” and under both currently and in years past.
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.