It's not a requirement to be quick, but it is becoming more and more useful in the modern game. For example, see the top players of the new generation of centre-back - Pique, Chiellini, Silva, Hummels, Jones - and you can see they all have at very least serviceable pace. With top-level traditional target men strikers dying out and being replaced by more complete, pacier forwards, centre-backs in general have to be quicker to compensate.
That's beside the point, however. The point is that one of our two centre-backs needs to be quick and have the positional awareness to cover, so that's either Jags or Smalling. The other centre-back can be free to push up and not have particularly notable pace, though Jones can shift like buggery when he needs to. Jones is a promising youth who has been in fine fettle all season, and allows us to play a high line without exposing one of our centre-backs for pace. Terry, on the other hand, looks wholly uncomfortable in a high line, either forcing us to play deep or him to underperform.
We're exactly one month into the Euro year and someone's already mentioned passion in regard to an England player.
Sigh.