There is not enough information supplied and the question is too general to approach a performance issue.
Searching https://support.emc.com for "Avamar client performance" is a good start. There are a lot of knowledge base articles that can help you narrow down and better define the issue. Start with https://support.emc.com/kb/335209
Particularly for the difference between Avamar VSS backup for Active Directory Domain Controller that take longer than a regular VSS backups used for BMR check this https://support.emc.com/kb/514283
Hope that help