Hopefully we can get an answer directly from Symantec on this soon. I'm having the same issue - similar setup to Jorgen, except for direct attached storage. Currently I have 140GB of data that, when backed up directly, runs at 3,140MB/min. Once I turn on GRT, it drops all the way down to 105MB/min. The job that used to only take 1 hour to complete, I had to cancel after 12hrs because I can't run the backups during business hours - and the job was only 50% complete.
I'm planning on adding four more VM's which, once configured, should add another 100GB to my backups.
I can't live without a backup of these .VHDs, but I can't live with a 2-day long backup run either. (I have two more Hyper-V servers that I want to add in eventually, but with well over 1.1TB total in VHD files alone.....(let's see 105MB/min = 6GB/hr; 1,340GB/6 = 223.3hrs; 223.3hrs/24hrs in a day = 9.3 days to complete my backup??)
The only other option I can see would involve shutting down the VMs, copying the .VHDs, then running a files-only (no GRT) backup of the .VHDs AND the individual files in each VM. The issues with that are 1) I can't afford to shut down the VMs every day just to run backups and 2) I don't have 300GB of disk space lying around just to use as temporary storage.
I understand Ken's point, but with Exchange, you are running a block-by-block copy of data while the store is still mounted and in use. With this, you're working off the snapshot image of the VHD and mounting it in a manner that the individual files are visible. In all essence, you are running a second instance of your virtual machine off-network. The backup of the individual files at this point, again as Paul pointed out, should "scream". Granted I understand that you are still creating multiple backup files of the same data (Once for the snapshot, once to copy the snapshot's VHD, and then once mounted, copying the data from the VHD).
Even adding in time to mount and unmount the VHDs, I can understand it taking 4 or 5 times as long as just the files themselves - but there is no way that I can understand why it takes 20 to 30 times as long.