Netback up running very slow

As others have already explained: 

There is no 'magic fix' for slow performance.
You need to find the bottleneck.

The 1st step is to test read speed from client disk. If this is slow, there is nothing that can be done from NBU side to make it faster. Fragmentation on client disk is normally the biggest cause.
See http://www.symantec.com/docs/TECH17541

Step 2 is to test network transfer rate.
TCP Chimney has been known for causing slow backup performance.
Switch this off on backup server and clients.
http://www.symantec.com/docs/TECH60844 

Step 3 is to test the media server's own ability to write to tape.
See http://www.symantec.com/docs/TECH19206

Use this doc - although it says 6.5. The principals remain the same:

• TECH62317   NetBackup 6.5 Backup Planning and Performance Tuning Guide (updated March 12, 2010) 

Some more reading matter:

 Overview of NetBackup performance testing. http://www.symantec.com/docs/TECH147296

About tuning Windows for NetBackup: http://www.symantec.com/docs/HOWTO56210 

PLEASE do not ignore advice to upgrade. 
NBU 5.1 ran out of support many years ago, plus some 'bugs' and 'features' caused slow performance:
Unpatched version of 5.1 had a 'hanging bpfis' that caused long delay at start of backup.
'Client job tracker' was enabled by default, causing slow performance.