It’s RoonOS performing badly. Your Nucleus Plus is fine.
I have a same-ish setup as you, Nucleus Plus, 16Gb RAM, 250Gb m.2 for OS and an 8Tb SSD, both drives Samsung. My media drive is full though.
Have a look here:
You’ll see that with Windows it is performing MUCH better. So wasting money on hardware when the operating system can be optimized, no thank you.