When I had my libraries on a relatively slow NAS (not supported, but worked for me) I put the metadata.db for the libraries on the fastest local drive using symlinks (Ubuntu Linux). It is possible to do the same in Windows. Perhaps you could do the same? But don't try it if you don't know what I am talking about, and definitely not without a recent backup.
One problem with editing metadata is that some edits stay in metadata.db. But some edits means that the folder that is used to store the actual book also has to be updated. If you change author or title of the book, then the folder has to be renamed. So moving metadata.db to a faster drive may not be a complete fix. And for large libraries it is slow editing and moving between fields and books anyway, even using a fast drive and plenty of memory.
Add more RAM if you can. Another option, if adding memory is not an option, and you have USB 2 or 3, is to buy a big superfast USB-stick and activate ReadyBoost on it. That might also help calibre.
http://en.wikipedia.org/wiki/ReadyBoost
http://en.wikipedia.org/wiki/NTFS_symbolic_link