What firmware on the DNS-320?
Is the DNS-320 directly connected to the computer you are running the test from or does it go through a router?
If computer and nas connect through a router does it have GB ports?
Again, if connection is via a router or switch what other activity was there on the network?
As you can see there are several things that can cause problems with speed tests, even on GB lans. Somewhere on the forum are the results of tests using various firmware levels with the latest firmware being the slowest.
The other thing to consider is, does the test speed really matter? In other words, do you have problems reading and writing to the nas in day to day activities? Yes, the read speed is low but there are other factors that may be causing that such as the network card in the computer, even the NIC drivers can cause a problem.
I have seen a case where updated NIC drivers actually slowed down network activity which we regained by restoring the older drivers.