Lots of guys run 4.3. But he has yet to demonstrate that he can run routes or catch the ball. Or for that matter that he can be an effective runner in the NFL. There is a reason the guy was primarily a special teams player in college and for the last 3 years in the NFL.
He did have a good preseason as a runner this year so there may be some upside to him as a RB, but there is no indication that he is particularly good in the passing game.
Also, Shaheed looked good on the Jet Sweep last year so I think there are a lot of YAC yards that he will get in the new system that is designed to create YAC. Same for Olave. And, if I'm using a RB in the Slot to get YAC, it's going to be Kamara who has the ability to be the best Slot WR on the team. Except he a really good RB.