Brown and silver leghorns are quick. Good looking birds too.
That being said if you watched the You Tube video in this thread (and you say you did) you will see why no chicken is safe from hawks unless they are in a hawk proof run. Did you see how fast that hawk came out of the trees and around he building, no chicken can move that fast.
Many say have plenty of places to hide, well these hens were hiding in the building and the hawk just went right on in like he owned the place. If they spot a hen run under a piece of wood or a bush they do just what this hawk did, go to ground and chase it down. Remember hawks can see mice on the ground from hundreds of feet in the air, a big ole chickens sprinting, flapping across the yard to hidey spot isn't going to get past them. I realize the chickens were "sitting ducks" but in open range the hawks will eventually get a chicken, fact of life if you free range. My experience is once they get one they just keep coming back.