Performance Tuning is a lengthy process. There are no 'quick fixes'.
You firstly need to understand the capabilities of each component in the data path, starting with Client disk and OS, NIC, the traffic on the network, the NIC on the media server, PCI bus, number of tapes per HBA, and lastly the tape drive or disk on media server.
A lot of people will tell you to start with Buffer settings.
I believe that large buffer sizes will actually slow down performance when clients cannot generate data stream fast enough and network cannot transfer data fast enough. The (large) buffers (buckets) on the media server will simply sit an do nothing while it is waiting for buffers to fill before sending it to the tape drive.
The Backup Planning and Performance Tuning Guide http://www.symantec.com/docs/DOC4483 will tell you how to test each component in the data path and how to decide when and what buffers should be adjusted.
There are lots of reading matter available on design of backup environment and tuning.
Overview of NetBackup performance testing.
http://www.symantec.com/docs/TECH147296
Key performance considerations for NetBackup 7.5 master servers
http://www.symantec.com/docs/TECH202840
Designing your backup system
http://www.symantec.com/docs/HOWTO56078
Reasons why the data throughput rate can be slower than the theoretical maximum when backing up to or restoring from tape media or disk (B2D) & how to troubleshoot or improve backup performance.
http://www.symantec.com/docs/TECH8326
Improving NetBackup backup performance
http://www.symantec.com/docs/HOWTO44836
NetBackup™ Architecture Overview
https://www-secure.symantec.com/connect/sites/default/files/b-whitepaper_nbu_architecture_overview_1...
Enjoy reading!!
