Fra Roon:
With a small 1500 album library and a headless use case on a dedicated device, a very modest piece of hardware will run Roon really well.
If you're in the middle, say 3,000-5,000 albums, something similarly midrange--on par with the current mid-range Mac-Mini (Core i5, 2.x Ghz, SSD) will give a great experience. If you've got a 4k monitor and 20,000 albums, look for a big scary Core i7 with plenty of ram, an SSD, and discrete graphics. Most people will fall somewhere between those extremes. Not everyone cares as much as we do about "perfect" performance either. That makes it really hard to talk about what "suggested hardware" should be to everyone at once, but it should give you some idea of what we consider "extreme" and where you fall.
Some more specific points:
Flash/SSD storage is a big win. The metadata and artwork databases I was talking about are sitting in your home directory on your boot drive. If you have a small amount of music (lets say, less than about 1500 albums), it barely matters, but as your collection creeps up towards 5,000 or 10,000 albums or more, storage performance begins to matter a lot. As a very rough estimate, plan to set aside about 2gb of disk space on your boot drive per 1000 albums. Expect the app to use somewhat less at first, growing over time as we expand music metadata capabilities and improve artwork quality.