When I connect to the DIR-812 directly (LAN or wireless) and transfer a large file from an attached USB drive the transfer speed is around 15-25Mbps. However, if my system is connected to a DAP-1522 (bridge) and then to the DIR-825, My transfer speed caps at 10Mbps. I tested my transfer speed between the DAP-1522 and DIR-825 with a machine at each end I'm getting around 45Mbps.
I hope this performance issue with SharePort will be addressed soon.