I looked in the .kobo/kepub folder on my Kobo Touch this morning and I see a file named a4407fbb-e573-4272-8ff2-4e9f7912ff4f which is 17MB and date/time stamped at the appropriate time to be the book I'm experiencing the slow endnote viewing. (I originally said it was a 9MB file, but that turned out to be the Adobe DRM ePub version I downloaded to my PC.)

I will say that this book does not seem to have been formatted well by the author/publisher, and therein might lie the problem. As we know, some authors use well formatted and thought out HTML and CSS that works well in an ePub, while others don't have a clue as far as how o create digital books like ePubs. I have not attempted to peak inside this kePub as it is DRM protected, but I did notice that the Adobe DRM ePub version is smaller at 10MB and the MOBI version (bought through Amazon) is smaller still at 5MB.

Size Comparisons:

kePub = 17MB (from KoboBooks)
ePub = 10MB (the Adobe DRM version from KoboBooks)
mobi = 5 MB (from Amazon)
converted ePub = 2MB (converted mobi to ePub via calibre)

The ePub and especially the kePub seem extremely bloated to me. This often happens when authors use tools to quickly generate HTML from other documents. My guess is that a 17MB ePub that looks essentially like a 2MB calibre generated ePub must contain a LOT of superfluous HTML/CSS and perhaps this is so convoluted that it is slowing down the display of the book.

Also note that this book is a scholarly work filled with hundreds of hyperlinked endnotes documenting her sources of information.