WDC (Wireless Device Connection) Mode behaves just like a USB cable, A PUSH from Calibre. That supports Multi-selected (in Calibre)- Send to Device.
OTOH Content Server (or cloud store) is a PULL... one at a time
I second doing small size batches: Flash storage takes time to write, the DB entry needs to be processed. Too much, Too fast, just make a chance to choke
