I think Index is common for all books, not for every single book. So it need to reindex all if you add a lot of books at once. If you add a book or two it may use additional index files and search first in the main index then in new index files. And somewhat triggered to reindex all when needed. This is my opinion - I do not have Kindle source files.
I also give it to charge for a 2-3 hours when adding more than 5 books at once.