It'd be nice to be able to customise what gets put in the metadata.calibre file which is sent to the device.
Obviously, some fields are required to be in there, but unless there's a pressing need why include custom columns and all of their details (like the formula used to generate them)? And why embed entire thumbnails in the json structure? Those things just seem to produce file bloat for no apparent purpose.
So, yeah, is it possible to allow the user to select which (non-core) fields are included in that file, and to switch off by default the sending of anything non-core to it, to reduce file size and hence transfer/sync time?
I've raised a ticket here:
http://bugs.calibre-ebook.com/ticket/8001