Of course 500 books compared to 30 is a huge difference, but I was replying to the OP who indicated it takes his wife about a minute to add a book to a category. I said I'd added all mine in a couple of minutes.
It can be a slow process if you do it one book at a time to a collection, but if you do it by collection, and add a host of books at a time to one collection, it's a lot quicker immaterial of how many books you have.
Incidentally, I have all my books organised nicely in Calibre, far easier than having them all on the Kindle.