Hello,
Usually the reinstallation should not cause this phenomenal change in the backup speed. Had you done any other changes on the system?
You can check the following technote that might be useful:
Reasons why the data throughput rate can be slower than the theoretical maximum when backing up to tape media
http://support.veritas.com/docs/231488
Also check the following:
What Backup Exec settings can be modified to reduce the amount of time it takes for a backup to run?
http://support.veritas.com/docs/249090
Revert with details in case you have any more questions on this issue.
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Regards.