I put the registry key on the server being backed up and it appears to be working now.
The first treewalk can be disabled by setting this registry variable on the Disk Agent host:
HKEY_LOCAL_MACHINE\SOFTWARE\Hewlett-Packard\OpenView\OmniBackII\Common\Parameters
Value Name: NoTreeWalk
Type:REG_SZ
Data:1
This new value is read automatically when the next backup starts.
Thank you!!