Only issues with one volume? Do you backup other volumes from same device that backup ok?
What is the make-up of data on volume? (e.g. size, number of files etc)
Are there any disk issues on NDMP device? We had an issue where performance for one volume dropped dramatically - the length of time trebled from one week to the next - it was eventually determined that there was a 'faulty' disk in the containing aggregate that was being continually thrashed (constantly over 80% utilisation/access) & when this was replaced problem was resolved.
Activity on filer when backup active? NDMP gets a 'back-seat' as far as resources are concerned if there is a lot of user activity.
TechNotes - found a few (altho' they may not be relevant to your situation), there may be more:
Number of files has a large impact on performance. If you have 500K files, not too bad. If you have 20M files, takes a lot longer.
Are you doing DAR restores? I've seen issues with some versions of OnTAP prior to 126.96.36.199 that have an issue on restore where the NDMP header is read slowly. If you have a lot of files, it can take a day to read the header. Then it speeds way up and you can restore files at a healthy clip.