What is the souce and destination?
Local disk to tape?
Local disk to remote disk?
Counter intuitivly, sometimes you can get increased performance with smaller number and size of buffers, since it waits until they are filled to send them.
There are some great posts/articles here about sizing these.
You tags indicate windows - setting large frames on the backup network should show an excellent improvement.
Please share job detail of problematic backup. NetBackup is application and OS is resources provider.
I suggest first look into the resources (Client disk, Network etc).
Thanks & Regards,