It's still crazy to me how so many NFL front offices treat the combine more important than 4 years of actual footage of prospects playing football at a high level.
Jennings will never have to run a 40-yard dash as an NFL WR. He will have to run routes, fight off cornerbacks, juke defenders, catch bullet passes, and break tackles while wearing full pads and a helmet every game. He's a lot better at those things than the 40-yard dash.
Jennings probably wins the 40-yard "carry a defender on your back" dash, as well.
I'm not saying speed isn't important, but it's much more important for some players than others. Jennings isn't a speed WR. That's not how he makes big plays.