Hi,
Which version of SQL are you using?
For test purpose disable all the antivirus software and third party applications, before running the backup job.
Is local backup working fine? Does this issue occur only during SQL backup?
Also refer this technote:
http://support.veritas.com/docs/190828
Test backup to backup to disk folder and observer the throughput!
Please refer to the following technote :
http://support.veritas.com/docs/237444
http://support.veritas.com/docs/249090
http://support.veritas.com/docs/231488
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