Using just rm -rf by itself won't actually do anything. You need to give it something to work on. If this is based on a find command, a driving file or something else can you show us what you are actually using?
You may get slightly better performance from putting multiple files on the command line, but not necessarily noticeable. if you have great chunks of data to delete, it might be better to split up your NAS provided storage and have separate filesystems. That way you can follow the suggestion of hicksd8 in post 2.
How is your data arranged?
Robin