I may be completely wrong about this, but wouldn't the battery go down quicker the more books you have on your device ? I would of thought that the more books you have the more power it takes on boot up to index the files. As I said I may be completely wrong, it wouldn't be the first time.
I have roughly 100 books on my (internal) SD Card and searching is pretty quick, certainly not any slower than I'd expect. I'd of thought the more books you have the slower any search would take, it may even be slower if they're on a different SD Card.