The scene doesn't seem to matter. So rather than explain all of the specific lighting, camera, material and geometry settings of a complicated scene, I figured I'd create a new, blank scene with default settings. That way, it should eliminate a ton of variables. I added a cube from the KeyShot "Add Geometry" menu, and left it with the default diffuse material. Sent that to NR, still only 10% from the CPU.
From there, I ran some other tests using the same simple scene. I turned on Performance Mode and sent that to NR. I tried some with different resolutions, from my standard 5120x3072 (5k at a 5:3 aspect ratio), down to 1000x1000. I changed the output file types, using png, jpg and tif. I also changed from product mode to interior mode. All with the same ~10% utilization.
I'm sure I'm missing other things to try. Let me know, and I'll run some more tests.