I think some people are missing the point.
Yes, Walker is one of the fastest fullbacks in the world. He's also below what we need in almost every other area. He's weak in possession, offers very little going forward, makes too many errors and gives the ball away trying simple passes regularly. I'd rather have a very fast and well rounded right-back than an extremely fast but average one.
Also I don't know where the idea that Cancelo is slow is coming from- he's easily quicker than Mendy, Zinchenko and Danilo