[crane3]

I think the slowdown is at the Kindle. Remember that each book sent needs a folder creation for the author, a sub-folder for the book then the files for the book in the sub-folder. Each step may require a security/permissions check depending on the tablet os version & cpu.

I was delighted that my Mediapad M5 was able to write to the external sd nearly as fast as the Fire HDX, sometimes. Then the Tab S2 & ASUS Zenbook have the slowness I expected in adding books to the CC book lib on the external sd.

The Likebook Mars is even "slower than molasses" in adding books to the CC lib on the external sd..

edit: Also Calibre does some stuff before actually sending the book as seen when selecting the "job" view. about 1 or 2 mins past after several steps before the book is sent. Do not know the additional time/steps needed when converting on the fly.

Haven't tried to see how fast it is on the ipad 6th gen for adding books but that is a different CC & everything is sandboxed.