No, it has to use the same database as Joomla. However, the amount of data held in the nBill tables cannot have any effect on the rest of your website unless you are using the user subscription mambot - even then though, you would have to have an awful lot of subscribers for it to make any noticeable difference. It might be that running Joomla in legacy mode is slowing things down though. nBill 2.0 will not require legacy mode to be on, but that is still a few months away yet.
