I’m not that familiar with the hardware specs of the Roon Nucleus, and while I’m a big proponent of having a beefy machine running Roon Core (like Erno’s), I find it very hard to believe that an intel Core i3 machine would struggle with Roon Upsampling to DXD. 
I run RoonServer (no UI) on a dedicated machine with a 6-core Intel Core i7 2.3GHz/16G RAM. I just checked its CPU utilisation while streaming with Roon DSP enabled and Upsampling to DXD, it was only 2.96% sustained. Although I’ve not tried it myself, I’ve read of even a Raspberry Pi4 with embedded HQPlayer being able to handle PCM Upsampling to DXD rates.
Unless Roon ROCK on the Nucleus is incredibly inefficient, my guess is something else is going wrong in your setup, not raw CPU grunt of the Roon Nucleus.
How big is your Roon album/track database? What are all the elements involved between your Roon Nucleus to your Rossini? Is it just one Ethernet Switch, or more? And you mentioned a NAS, how’s that connected to the system?