The router should be a non-issue, the LAN ports are a simple gigabit switch on that router. I don't know if that switch supports jumbo frames for gigabit, but even non-jumbo frames should do a lot better than that.
I just copies a 696mbyte file to the DNS-321, the copy took a bit less than 40 seconds for a 17.4mbyte/sec rate. Reading is a bit slower for me, don't know why.