Hi,
Refer to the following steps which may help in improving the performance.
The Antivirus software is configured to scan files when accessed. So, each time Backup Exec attempts to backup a file, the antivirus software scans the file and slows down the backup.Try backup by disabling the scanning feature of the Antivirus software in order to return the backup to normal speed.
Please ensure that the Virtual memory is properly configured.
Refer to the recommendations given in the following technote.
Technote: What are the Virtual Memory recommendations for Backup Exec for Windows NT/2000?
http://www.seer.support.veritas.com/docs/233196.htm
Technote:What Backup Exec settings can be modified to reduce the amount of time it takes for a backup to run?
http://seer.support.veritas.com/docs/249090.htm
Refer to the following technote to determine the hardware and network parameters to be configured to potentially improve the performance.
Technote: Reasons why the data throughput rate can be slower than the theoretical maximum when backing up to tape media.
http://seer.support.veritas.com/docs/231488.htm