Williams is probably our fastest guy now, with legit 4.4 speed, but that's assuming he's back at that speed following the injury. I can't say what Moore is running these days, but at his pro day I see him listed (DraftScout) at 4.49. They're usually pretty reliable, and that's far from an elite time.
Again, I'm not saying it's a big problem, but at some point it would be nice to have a burner to really push opposing safeties back.
I think what's more important than WR speed is the ability of the QB to make all the throws accurately and crisply to all parts of the field. Kaep has that kind of arm strength and delivery. Alex did not. Kaep will use the whole field then WR speed is not as critical.
Plus our guys are not that slow. We have a bunch of guys in the 4.45 to 4.55 range. If you get jammed at the line of scrimmage that will easily knock off more than 0.1 seconds.