Don't now why one server backs up at twice the speed of the other (is one local to the media server and one accessed over the network?), but Mailbox backups (as done by Veritas/Symantec) are inherently slow. They also often present problems when one tries to restore from them. On top of everything else, they are totally unsupported by Microsoft.
One last comment - When/If it comes time to do a disaster recovery of Exchange, brick level are worthless.
With all this in mind, are you sure that you really want to do Brick Level backups in the first place. With stores level backups and a Recovery Storage Group, you can get anything back that you want.
For a more indepth discussion of brick level backups, see http://mail.tekscan.com/nomailboxes.htm it is a little dated, but all the points are still valid.