Sending can be slow sometimes, but it also depends on the size of the files. I sent about 650 files to my device and they too kabout 4 gigs of space (some 100 megs lrf in there). First calibre seems to manage the files in the temp dir and then send them to the device. The first step takes maybe 30 min or less and then it starts sending.
Main problem with sony 505 or maybe even 700 (haven t tested on the later), is when you send such large files the collection data gets corrupted and all that remains is the number of books. There is a tool on this forum that allows you to create the xml so it shows the correct collections. Have yet to try it, but it might be a good option for sending large files!
Edit: Here is the software that creates the xml :
https://www.mobileread.com/forums/showthread.php?t=29217
Might be the only solution to keep collection data in! (could maybe be a nice idea for a pluggin for someone who knows python.)