Slow backups are always a concern, they load the netbackup system and put all plans in a jeopardy.

These technotes are all related to troubleshooting slow backups, take a look at all of them before beginning a long journey

http://seer.entsupport.symantec.com/docs/232929.htm

http://support.veritas.com/docs/240607

http://support.veritas.com/docs/267253