It's a matter of how much of a lead Jaylon had and how far he had to go to get to the endzone. Jaylon admitted he slowed down which turned out to be enough to be caught from behind.
Yeah, he had about a 6-7 yard lead, but as a larger, slower guy he doesn't go from 0 to full speed as quickly. If you will notice, the majority of the ground was made up in the first 6-7 yards - that's the initial quickness a smaller, faster guy has.
And, again, he just doesn't have the same raw speed as a 4.3 guy. No LB does. In addition, Smith is running with an eye on potential tacklers and how Gregory's block will play out and if he will need to make a cut, and McLaurin is running on a bee line to Smith.
Ultimately, Smith is clearly pushing himself and running hard, and there is no reason to think he was doing so at a notably slower speed than the majority of NFL LB's. In fact, if you watch the video, Smith was clearly moving faster than everyone else except McLaurin.