Will Roon work on new QNAP TVS-873 NAS with 16GB Memory?

Doubtful. DSD512 is about 50Mbit/s. Not even remotely a concern for 1GB Ethernet. Your 5.1 DSD256 will be about half that. If you have a Wi-Fi link in there, then maybe, since Wi-Fi is often crap (especially with defective-by-design hardware that only supports 2.4GHz). But if its all wired, then I really doubt its a network bottleneck.

DSD256 is a pretty big hit to CPU on the Roon Core, however, if it has to do any DSP work (like if it has to convert it to PCM for your endpoint). Your screenshot showing CPU at 49% I assume means your NAS has two cores and Roon is capping out one of them at 100%.