Maximum single-threaded speed in the core is the main determinant in user experience snappiness.
Multiple cores helps with running more zones in parallel with more DSP and will complete stuff like audio analysis faster. I wouldn’t go beneath 4 cores/8 threads for this use case. You could do more, but added value will diminish depending on how much you value those aspects.
I understand the preference for Linux (and wouldn’t want to run Windows if I could avoid it), but in your shoes I’d do an A-B test and come to a personal understanding of what the differences are before making up my mind. It costs nothing but a few hours to install Windows without activating and learn the situation.
Here’s my feeling: Despite ongoing improvements to mono and more and more code from Microsoft.NET being merged in, Microsoft’s garbage collector is still better at handling the massive in-memory object count that comes with a library of that size. For most libraries the gap is negligible–there’s just some design choices in that GC that start to become more painful around 4-5mm objects on the heap, and really large music libraries can push things that high.
I don’t think Microsoft’s GC is ever going to make it into Mono. I think instead they’re going to keep making .NET Core better until Mono is displaced. At some point they will reach a point where we can switch to .NET Core. I have a hard time imagining Microsoft doing that and us being able to start that effort until a least a year from now, but it could be more depending on whether .NET Core 3.0 is actually viable or we need to wait another iteration.
In other words: if you didn’t have a strong preference for Linux, Windows/Microsoft.NET is the safer recommendation at this unusual scale. Since you do, it may be worth the extra few hours to try it both ways and see, just in case the Linux performance on a fast machine is good enough and you can avoid the Windows headache.
As for other hardware choices:
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High-End Desktop vs Server CPU: High-End Desktop. Especially now that there are Threadrippers and i9’s, there’s no reason to buy a Server CPU for something like this unless you need Server motherboard features like redundant power, remote admin, etc. 4 Cores/8 threads is probably fine. You could go up to 6-8 cores especially if you see yourself running more zones/dsp.
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Intel vs AMD: We test, develop, and ship hardware based on Intel, so there’s some passive bias. That said, AMD might get you more performance for the same $. I have a very, very fast AMD machine here (built for batch processing/machine learning) that runs Roon very well. I wouldn’t worry about it too much.
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NVMe SSD is a good idea. Larger sizes tend to be faster, too up to a point. Samsung 960 Pro is very good.
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Definitely store the music on local (USB3 or SATA) drives and stop watching the NAS. This will help more than you think.
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For 640k tracks I’d consider going to 32GB RAM. Even if Roon doesn’t use it all, you’ll benefit from extra filesystem caching + more breathing room.
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If you wanted to try ROCK, NUC7i7DHNE is the best NUC right now. You can hang USB drives off of it and it will support the storage. It can take an NVMe SSD and 32GB of RAM. So it could work.
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If you want to build an even bigger machine, run Ubuntu or Windows. Getting a shell on ROCK is more difficult if you want to do sysadmin stuff like setting up cronjobs, rsync, etc. So that may push you in other directions.
That covered what I can think about right now. Any more questions/clarifications, let me know. I’d like this build to be a success for you.