The time taken to do the "send metadata to device" job does not depend on how many books you send to the device, but on how many books you have on the device. Each time it runs, whether it is when you connect the device, send or delete books, it will process the details in the database on the device for all the books to update the collections and series info if you have these set. It also updates the file metadata.calibre that calibre uses to assist in connecting the books between the device and calibre library.
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