I said could, not is.

I've got an EPUB here of the larger persuasion that takes ~10 s to load in Nickel and ~5-6 s in KOReader/crengine, which is the general trend in my experience.
But indeed, Nickel is surprisingly slow to turn pages. It seems to take a small eternity, although tbh I hadn't really noticed in normal use. I wonder if maybe it's an artificial delay, e.g. to support double taps, to prevent accidental double page turns, or to avoid potentially ugly looking overlapping refreshes?