Thanks for the tests and for the numbers.
An average of .6 seconds per book seems right for a library that size. We put a lot of information about each book in the database to make grouping and sorting fast. The time to remove that info for a book is related to the number of books in the library. For small libraries it takes around 300 ms. My guess is that in your experiment the time started at close to a second and got faster as the library got smaller.
As for sending, for my lib I see an average of around .7 seconds. My guess is that on average your books are slightly bigger than mine.
Note that the (long) db update is overlapped with the network traffic, but it does put a lower limit on the per-book time of around 300 ms, longer if there are lots of tags in each book.