The big difference between my 700 and my 500, when loaded with lots of books on the memory card as well as in memory is that once the reader indexes all the books (or whatever it does when first disconnected after loading a lot of books), it's just as fast as when there are only a few books in the main memory of my 500.
With my 500, when I used the memory card and put a lot of books on it, it would take a long time to open a book I was already reading and a long time to put a bookmark and a long time to remove a bookmark.
The 700 exhibits no such behavior -- the most books I've put on it so far have been 2040, mixture of large PDFs, tons of LRFs and about 400 TXT files.
So just be patient when the little thing starts spinning and go do something else and wait -- once it stops it'll be worth it.
And with the alphabetical index down the right side of the library listing and the ability to go to a specific page number it's easy to get to where you want to be in such a large list, unlike the 500, which was a royal pain to sort through.