Hi,
you should start to check the performance using a Nulldevice. If possible, use multiple streams instead of 1 DA per Mountpoint. To check the speed online, you might use omnispeed - see http://www.data-protector.org/wordpress/?s=omnispeed
You can also check if Disks in VM are fast enough to deliver the data. This can be tested using vbda - see http://www.data-protector.org/wordpress/2013/10/performance-test-disks-data-protector/
Best regards
Daniel