It differs per book. If it is a big book wil complicated chapter structure it does take a "while". I admit if no book is open and the device wakes up to the home screen it is indeed pretty instantaneous, maybe one or 1.5-2 seconds... but I have been annoyed with waiting for the book page to load in the past
My Glo is probably also a bit slower than the Aura.
Yes that weighted average sort of system does sound like it makes sense... I'm still curious to know how the Kobo calculation actually really works though. Just so I know how to use it best to get the cleanest possibly statistics. I'm a relatively slow reader and am trying to increase my speed, so I'm picky about how correct the statistics are