I have original Kobo. I concur, it sounds like you may be reading an unusually-formatted book.
With the previous software version it was slow to turn pages. This actually was not a problem as I quickly got into the rhythm of clicking with about 1/2 to 1 line left on the page. New software is so much faster, I find it's just fine, nowhere near painful. And the wifi is reported to be even faster.
That being said, page turns to a new chapter are quite slow on the original Kobo. Your book may be hitting that problem on every page.