My DS212 (a wee bit faster than the 212j) on gigE to a fast PC - like all NASes, is limited by file system, SMB and TCP overhead. For certain jobs, it sometimes gets to 60-80MBps (x8 for mbps); but normally for big files it's 40+MBps (320Mbps). You should see at least 80% of these numbers on your DS212j, I think. It might be PC-limited.
Tests I've done in the past show a remarkable difference in LAN throughput (no disks used in the tests) between fast and lesser CPUs, for Windows. The fastest I have is a quad core I5. I think it's huge overhead in Microsoft's IP stack and moreso NTFS and SMB.
That said, it's rare that I do Gigabyte file transfers. Most common are my drive imaging (Acronis) of several PCs), yielding a 40GB file for each. In that case, the backup utility is the constraint - as it works to eliminate empty blocks and so on.
But still, I'm quite satisfied with the cost/benefit of GigE LAN and the NAS. And $20 Acronis 2015 is finally improved in ease of use. A staple for me is Centered Systems' SecondCopy and versioning for my (professional) work in process so I can easily fall back if need be.
The CPU overhead is pretty tiny. My z3740 based tablet can easily haul full gigabit over a USB3 GbE adapter. Only storage limited (so if I use the eMMC storage, I get ~110MB/sec read from the tablet and 40MB/sec writes from the tablet to my server. If I attach a USB3 500GB HDD using a hub so I can use the GbE adapter at the same time, I get about 108MB/sec reads and writes, which is the limitation of the 2.5" disk in the enclosure).
The issue is the NAS CPUs are generally very, very, very low performance. Just like router CPUs are. Until recently most routers were using processors about the equivalent to 25-40% of an iPhone 4's single core Cortex A9 running at 800MHz. Even these days most routers are only maybe a Cortex A9 dual core processor at around 800-1000MHz or some non-ARM based processor of roughly similar computational capacity. They are highly non-optimized for storage based on their system buses (IE they often don't have dedicated XHCI controllers, or SATA controllers, but use bridge chips to the native, and slow, system bus).
NAS are sometimes in a bit better shape because they tend to have better system buses, but they are still often pretty slow. A few years older ones, like the 212, IIRC use a Kirkwood dual core 800MHz process (I think in the 212j and the 212 uses a 1000MHz variety, I think). These are about the equivalent to maybe 20% of the processing power of my tablet, or maybe as fast as an iPhone 4s on a good day, with a tail wind.
This is partly to say the appliance processors are advancing rapidly, but even most NAS still use processors that aren't even fast as a current smart phone, let alone a tablet, let alone even remotely close to a laptop or desktop. Oh, sure, some higher end NAS do. One of the issues of course is that there haven't been a lot of low power, but powerful processors until recently.
Of course they also want to shave money where they can. Hard to justify a $40 processor in a $100-200 product.