Pro-tip: if your machine is acting sluggish under heavy load (e.g. when Arq is running a backup), free up some cycles by turning off the "Desaturate" filter in Isolator. This can make the difference between the Arq pegging an entire core and the
WindowServer process soaking up 40-60% on the other core, and the latter using a mere 2 or 3%, freeing up the rest of the core for user interactivity. This makes a huge difference to system responsiveness; so much so that I am tempted to leave the "Desaturate" filter off forever.