From what I've seen of him I don't think he's that slow.
But what will get him in trouble is if his challenges/interceptions aren't timed properly and if he misses it he will probably get outpaced. He's not a short guy he's around 6'2, and he won't be able to turn on a dime like let's say someone like Mascherano and even Umtiti.
But to his credit, he's only 23 years old so he has plenty of time to improve on his flaws.
I think we would neutralize pacy forwards better if we returned to high and intense press at least in CL games. At some points in yesterday's game we did that and Tottenham is useless without being able to pass around the ball freely. They are also a keep ball team, only much lesser than Barca. If we let them run at us, while we fall back, we're usually up shit creek.