Comment 1 for bug 1593027

Copying to library has to first serialize metadata, then import it into
the destination library. Importing directly makes use of the already
serialized metadata in the OPF file -- so it will always be a little slower than
direct import. However, most of the performance difference comes from
the adding books process having an optimized implementation for finding
duplicates -- I'll port that over to copy to library someday, but in the
meantime you could just turn off checking for dupes when copying to
library in Preferences->Adding Books.

And note that importing directly from the source library folders will
not lose any metadata provided that you run Library maintenance->Library
metadata backup stats and wait for the backups to be completed (this
causes the aforementioned opf files to be written out).