Netbackup backup speed and buffer size

Hi Guys

I have a server 2012 netbackup client with acelerator and tracklogs configured . The backup is working but looking at increasing the speed. I have a 20Gb network beween the server and backup server ...

Looking at the backup logs I see the  entry  ) waited for full buffer 4653 times, delayed 4854649 times   

The C :\ C:\Program Files\Veritas\NetBackup\zzz- NET_BUFFER_SZ says 256 ( Media server)

is there anyway I can reset theses setting to improve the speed? what the best setting taking into consideration my current settings 

Thnaks in advance guys