The problems may be due to many issues, cabling, motherboard chipset, HDD read/write speeds, network setttings, Proper NIC configuration, other network components (100Mb etc) as the network will likely operate at the speed of it's slowest cpmponent.
There could be many bottleknecks that you could try to identify, but in my experience even using high cost NIC's you won't get the max speed you expect ue to other limitations of other components.
I regularly see 20 MB/s transfer speeds withj my DNS-320 on an all Gb ethernet network, with Jumbo frames enabled and all NIC configurations checked to correspond with one another, before I made the move from fast ethernet (100Mb) my speeds were between 1 - 2 MB/s so seeing a x10 increase from 100Mb ethernet to 1000Mb ethernet, shows that at least the network is running optimally, it's just the HDD read/write speeds bottleknecking things IMO. as moving similar files even from one HDD to another on the same PC, usually results in transfer spees of approx 12 - 18 MB/s, depending on the file size...!! Likely a limitation of the mobo chipset..!!
I thinbk blaming slow speeds on the DNS-320 is probably not the cause in most cases...!!