Nazgul At Pelennor

[Eurytus]

Why not? The Winged Nazgûls can reach a really high speed. Look at the events that are explained in the chapter 'The Palantir': Pippin looks into the Palantir, immediately Sauron sends a Nazgûl to Isengard, and Gandalf and Aragorn see the winged Nazgûl flying over them while they are still talking about Pippin. The whole episode doesn't seem to take longer than one hour.

That is not correct. In The Two Towers Gandalf explains that the Nazgul was not going to Isengard in response to Pippin looking into the Palantir. He was responding to an earlier issue.