why there is SUCH a slow bandwidth transfer rate even with gigabit and CAT6.
Let's describe it as an incorrect expectation.
Many people expect a gigabit connection to be able to transfer data at - well - gigabit speeds, and fail to recognize that anything over hundred megabit speeds can be defined as gigabit speeds.
On a personal note, my network experience dates back to coax networks, and probably includes some museum pieces that you may never have heard of (ARCNet, twinax to name a few), folks like me who grew with the technology would have used 10 mbps coax networks on which 33% traffic was considered an overload.
Networks became a consumer technology when the internet became popular, and a 100mbit network could move data at 98 mbps or thereabouts, but in it's infancy, even 100mbit networks couldn't handle much more than the 33% mentioned earlier.
Enough history - your DNS-323 has the processing power to deliver around 400mbps to it's ethernet port, but when you start to add disk transfers and other housekeeping chores to the workload of the "flea" power CPU, it chokes, and the best you'll see is maybe 280mbps or therabouts, and that my friend is well over 100mbps and into the realm of gigabit speeds.
If you really want or need to see 800+ mbps storage, you can get it, but it's going to cost you a lot more than you paid for the DNS-323.
When you purchased, you opted for low cost, low power consumption, and probably did not recognize that low performance was a part of the bundle - EVEN THOUGH - the read/write speeds are stated on the outside of the box.
Mine chugs along, on a gigabit network, and I would guess averages less than 20~30 mbps on transfers, but it fills the need it was purchased for, acting as a backup storage device for my critical data.