I don't recall the last time I actually timed this, but I didn't nothing anything out of the ordinary when I did, and it was probably >= 4.2x (as in, epub was slow, but getting better, kepub was decent and also possibly getting better).
Granted, that was on a tame novel with sane chapters and sane-ish code, not a gigantic omnibus

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I do tend to notice and keep track of these things, to make sure KOReader still beats everyone, Kindle included ;p.
I'll run a test the next time I can and get back to you.
On the off-chance it's really book-specific, feel free to PM me a scrambled copy of something where you notice something weird

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