The time it takes to load an ebook depends on the technical characteristics and size of the ebook
itself, not how many there may be on the SD card. For instance, an .epub with large RGB graphic
images will take much longer to load. (Some may even have trouble opening at all.)
You could find that the first time a SD card with a large library is inserted, and the initial scan is
performed, it might take a couple of seconds longer than with a much smaller card. In normal use
there is no difference.
Having some special characters in an ebook's filename can also cause problems.
Having more than three levels of folders can cause problems, as well.
Otherwise, having a very large library on these devices is no problem.