Are you backing up to a removable disk drive?
I am having the same issue with slowness - only with Exchange - and I was told by a Symantec tech that version 11d, which now uses GRD backup technology has a bug in it and does not work well with removable disks. I was told to upgrade to version 12d, which has fixed the problem.
I was also told that since I recently purchased 11d I was entitled to a free upgrade to 12d.