kind of feel you are reporting the same issue: macOS Catalina interface laggy randomly while running Roon (High GPU usage randomly, Roon idle 20% GPU, sometimes up to 99% usage)
Same initial symptom, macOS interface was slow and kind of unresponsive, then you do cmd W or close or minimize (cmd H) then macOS goes back to life.
Is your Mac reporting 100% GPU usage while having the issue?