I think some of what you see in his highlights is that the competition is just overall pretty slow which makes him look really fast. I've seen an electronic 4.46 as his 40 time which is plenty fast.
He's playing both ways and doing kick returner duties. They showed him on the sideline and he was panting pretty heavy. I don't think speed is a concern for him.
He doesn't have the greatest open field speed, but he accelerates well. He is likely to play LB or Safety in the nickle here and that burst is mor important than a 40 time. That said, he is definitely faster than AJ