As far as I know, this is a known issue.
This issue got improved with every release since now, but does not get near the performance of the real backup job.
It has been way worse than now in former releases.
If you get any new information on this, let me (and all others) know.
I had a customer support call a time ago, and the only feedback I could get after talking to support, product management, and so on was, what I wrote above...