I just re-read. He blitzed and defeated bunch of BM fodders, no one notable:
Which, while visually impressive, doesn't dictate how is he faster than King?
Is Cavendish faster than anyone because of Hakuba stuff? Even though Robin could stop him.
Even pre-skip likes of G2 Luffy appeared very visually speedy. Or Zoro at Whiskey Peak (disappearing in plain sight).
But how do we scale him to being faster than King? He has only fucked fodders and stuff with his speed.