I agree, the issue is definitely not the actual streaming from the NAS. Regarding system load that’s a fairly easy exercise.
I suspect the issue is the process of keeping the database current with the NAS. Given an Intel Core processor, enough memory and an NVME SSD, the bootleneck is probably not within the ROCK. Any communication with the “outside world” is way slower than what is going on inside the ROCK. And an ethernet connection to a NAS is way slower than a USB3 connection.
In this thread it is suggested that the limit for Roon’s library is well beyond 400k tracks, but under which circumstances I don’t know.
Good luck