A "Library" in JRiver Media Center can be thought of as a database that stores the settings, playlists, views, etc. A Library does not store the actual media files, though. So, when you backup/restore a Library, think of it as saving/reloading the settings from before.
Since the music files themselves are not stored in the Library database, there is no need to worry about taking hours to copy the music to/from the NAS. The Library database itself is usually very small compared to the music.