I agree with others that it is a universal problem with eBook readers from all brands. I suspect it has to do with two things:

(1) The size of the book. Bibles tend to be large files.

(2) The complexity of the book and how it is formated. Bibles and other large books tend to have a lot of cross-referencing, and Bibles in particular have a lot of styles for formatting. There are cleanly formatted huge books which work well and horribly formatted small books that barely work. My guess is that most Bibles are simply too large for the publisher to do a good job of formatting them for eBook readers. They take shortcuts which result in horribly complex formatting (in the HTML) and that can drastically slow down the rendering of a book.

If you ever use a program like MS Word to convert a DOC file into HTML, you will see what I mean. Instead of creating a simple CSS stylesheet and applying one style hundreds of times with a class statement, they tend to envelop each occurence of the text with that style inside of span tags and manually apply the style. You wind up with a huge HTML file that is extremely cluttered with style statements. It takes much longer to render that convoluted garbage than it does to render a well and simply formatted HTML file.