The issue with slow page turns is one I've run into in the past.
I use rbmake and I believe this to be an artifact of the way the format/reader builds the page for displaying on the fly from text files (as opposed to the pre-paginated (?) format for the 1150). Due to this, it'll read through your file looking for paragraphs to break it up in before rendering it to the screen. Since your text file does not have delineated paragraph markers like an HTML file does, it ends up having to read up to wherever you are currently at in the book.
As you turn pages, it's reading further and further into the file, presumably loading all that into memory before giving you your page. If you try to jump really far ahead in a book directly (just hitting somewhere down the progress bar) you can probably get it to crash and reboot.
This only ever happens to me with text files that I've converted. The only way around it that I know of is to either get a HTML version of the same book, or convert your text file into HTML with sufficient paragraph's for it to handle. There may be some options in rbmake to avoid this with text files, but I'm not familiar with them if there are.