Backup job is very slow and KB per second is very low

Hi All

 I have an issue with NetBackup 7.5 on linux redhat 5 and the clients is solaries clients

The backup job is taking too long and the KB per seconds is very low on some solaries clients and the other clients is very fast and KB per second is too high and some times the same clients job stays green despite it is completed successfuly

Any suggests????????????