[NiLuJe]

I'll detail the "instantaneous" @ilovejedd mentioned:

For a random prose page, tightly fitted (i.e., very, very, very narrow margins and line spaces), in which I can fit 37 lines of text on a Forma screen, with Bookerly, rendered unhinted (hello, 300dpi ;p), but w/ ligatures, it takes CRe around 50ms to render said page, and KOReader ~3ms to blit it.
That's for a KePub (i.e., heavier DOM because of the extra Kobo spans).

All in all, less than 100ms. (Things get wonkier if you swap to a non-standard orientation, because we don't support hardware rotation for legacy reasons. Expect blitting to jump to ~100ms at worst, IIRC).

That's a crapload faster than both Kobo's renderers. That's also slightly faster than any of Kindle's three or four renderers.

The only trade-off is indeed the heavier cost at opening/re-rendering time (i.e., the toilet paper roll approach CRe takes vs. a chunked approach).

Note that the crappy ePub specification itself makes a chunked approach much more annoying to implement right. (i.e., there's a reason besides DRM Amazon never ever will touch ePub with a ten-foot pole: performance).