The first thing to recognise about a file transfer is that it starts as a datafile on a disk, and finishes as a data file on a disk and as such it is not reflective of network throughput - the transfer rate will be limited by the slowest channel through which the data flows on it's journey from one disk to the next.
When the DNS-323 was released, it was advertised (on the box) as being capable of speeds in the 20MB/sec region - I no longer have the box to quote those numbers, write was slightly under 20MB/s, read slightly over, and it will deliver those numbers as long as a) the network can handle the traffic levels, b) the other end of the transfer can handle those numbers, and c) the drives in the NAS are not fragmented.
Oh - you'll see higher throughput with larger file sizes - for example a single 1GB file will transfer a lot faster than 100 x 10MB files.
There's not a lot you can do to speed up the NAS, you just need to track down the bottleneck and remove that.
Search these forums for NASTester (or NAS tester) and you should find a link that you can download a test utility from - the utility was written specifically to test these NAS, and it works by creating a file of the size you choose, and then transferring that file in each direction as many times as you choose, and then averages the results.
A 100mbps network is capapble of handling maybe 9MB/sec or so, a gigabit network will double that, and jumboframe can net you another 50% or so, getting you in the 30MB/sec region - PROVIDED THE OTHER END CAN ACCOMODATE IT.
My desktop is an older Dell Optiplex (one of their business class products) with an onboard gigabit NIC, the disk subsystem limits it to just over 5MB/sec on a transfer from my NAS, but the same NAS, with no changes, over the same network will deliver just over 30MB/sec to an IBM server (of the same vintage as the desktop), also with an onboard gigabit NIC - the server has a much faster disk subsystem than the desktop.
The NAS is admittedly not very fast, but, as you can see, the problem frequently lies at the other end of the transfer.