Ridha,
What OS and NBU version are you using for your master server?
Also, same info for client?
There are several ways to improve performance from a client.
But it all depends on your environment and what is possible there.
How is your data organized and what type of data is it?
Can you 'granularize' the backup selection further?
If so, you could use Multistreaming:
http://support.veritas.com/docs/267291What is Multistreaming and how is it turned on?