The firmware revision on my Kobo Aura H20 is 4.38.23171. Page turn time is about 2 seconds. That is not lightning fast, but still tolerable for an old e-reader. I wonder if the speed of the OP's reader might be due to hardware, or the complexity of the epub file. My test was run on a Megapack book (1.4 MB, Achmed Abdullah). With these old e-readers, battery life is the most annoying concern.
A query of Google AI, to the question, "Does age affect the speed of an e-ink reader?", indicates that old technology could be the cause. There could also be an internal timer that is set when the reader senses that the screen cannot be cleared for a new page to be rendered. Is there ghosting on the display?
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