you never mentioned your drive. some drives are riplocked (only allow ripping up to a certain speed)
you can change this by using Media Code Speed Edit, checking "increase read speed" and then flashing the drive with the new firmware that you've saved.
be warned that modifying firmware voids the warranty with the drive.
also, I'm not sure if storage space is a limiting concern, but anydvd copies each disc "as-is." using another program to strip out extras and/or menus and just keep the main movie would decrease the size of each film and increase the max number of movies that you can store. (unless you wanted to keep the menus and eextras then by all means do what you're doing)
as far as i know, the anydvd ripper can not be opened in multiple instances. Anydvd can, however, decrypt multiple dvds at once. if you insert a disc in another drive, ther'es nothing to stop you from using clonedvd2 to write that disc to files on the hard drive. I think however I'm not positive that clonedvd2 can be opened in multiple instances.
the ripping speed may suffer if you do this meany at once especially if the destination drive is the same or if the drives are on the same controllers so it may still be more time efficient to do one at a time...but that's up to you to play with and figure out!