Not going to beat around the bush about the performance for Roon under OSX, its Bad. But I do have a temporary if you want to use the computer at the same time with roon running.
This sluggish syndrome as been associated to network connectivity not just on wifi but hardwired. This is not the case. Issue has been present is several releases or Roon for OSX and any iteration of 10.11.x through to 10.15.x
Temp Fix: - Command- H : Hide the windows of the front app
Let me explain this and why it works. OSX uses the openCL rendering system for applications. Rendered via the GPU and some CPU. Hiding the application takes it out of the “queue” of things for the gpu to work with. Bring it back, and bang - system performance tanks again. Hide it again and boom the computer is responsive. This leads me to believe that Roon isn’t using the GPU for rendering and the interface is strictly done via a software render.
I would highly encourage the app devs to review with they have based their rendering in a software or poorly implemented port using openGL in a opencl wrapper.