At the end of Book III, chapter 11: "The Palantír," in The Two Towers, Tolkien narrates the following (bold added):
[Gandalf speaking:] 'Away now, Shadowfax! Run, greatheart, run as you have never run before! Now we are come to the lands where you were foaled, and every stone you know. Run now! Hope is in speed!'
Shadowfax tossed his head and cried aloud, as if a trumpet had summoned him to battle. Then he sprang forward. Fire flew from his feet; night rushed over him.
As he fell slowly into sleep, Pippin had a strange feeling: he and Gandalf were still as stone, seated upon the statue of a running horse, while the world rolled away beneath his feet with a great noise of wind.
My question is about the bolded part of the text above. Earlier in the chapter, Gandalf states about Pippin's inquiry of him riding bare-back (more relevant bold added):
'I do not ride elf-fashion, except on Shadowfax,' said Gandalf. 'But Shadowfax will have no harness. You do not ride Shadowfax: he is willing to carry you—or not. If he is willing, that is enough. It is then his business to see that you remain on his back, unless you jump off into the air.'
'How fast is he going? asked Pippin. 'Fast by the wind, but very smooth. And how light his footfalls are!'
'He is running now as fast as the swiftest horse could gallop,' answered Gandalf; 'but that is not fast for him.'
So at a normal horse's gallop speed, the ride was "very smooth" and Shadowfax's footfalls light. When he accelerated at the end of the chapter, "fire flew from his feet" and the ride seems to have smoothed out even more, to the point of feeling like all was still as a statue between the riders and Shadowfax. Almost like the smoothness felt may be an inverse relation to how fast Shadowfax moves.
But that stillness was noticed "as [Pippin] fell slowly into sleep," so my question is was the stillness as a statue a dreamy perception by Pippin or a reality of Shadowfax's magic as part of "his business to see that you remain on his back" when he was moving at even higher speeds than a normal horse?