It makes it less likely that you end up with a partially successful import. Either everything will be imported or nothing. Calibre also works with a temporary copy of the database file. This may save you a lot of headache if the import failed halfway.
Calibre also works the same way when for instance bulk converting books or sending to device.
It is an approach that favour data integrity, reliability and safety over performance. I suspect that it is intentional.
Import fewer books in several installments and you should be fine. Optimally no more books than what can be held in the disk caches. Install more RAM to increase that amount. Make sure that the temp folder is on a fast disk with plenty of free space. I've actually put mine on a 8 GB RAM disk using the tempfs feature in Linux. Works fine, but plenty of RAM for disk caches should also help.
And seriously, how often do you import huge amounts of books? Go do something else during the import or take a nap. Perhaps read a book?
Last edited by Adoby; 03-13-2013 at 02:29 PM.