FWIW, a device-tailored CBZ in KOReader should be as fast as the screen can be driven, which is basically as fast as a Kindle can go, because it's also doing device-tailored files by default

.
(For a random unoptimized CBZ, you'll have to eat the (small) cost of an extra scaling pass, which will mostly always be hidden away, since we preload the next page).
I have no idea why this takes forever in Nickel, but it does, extra scaling pass or not.