cycles render on imac with GTX 680mx seems to stress

I’m just getting back into Blender after a bit of a hiatus. I have the latest official Blender, I have the latest nvidia drivers, I have set CUDA compute device in User Prefs -> System to my GPU, and I have GPU Compute selected as my Device in the render tab.

And I think I am rendering OK on the GPU: my test scenes / benchmark renders are in the region of what other people are reporting for my setup (early 2013 27" imac, quad i7 3.4GHz, 32GB RAM, GTX 680mx with 2GB vRAM). It’s just that whenever I render on the GPU, my CPU usage still goes to 100% as well and my whole system gets very sluggish. (Even though, as I understand it, a reported 100% in Activity Monitor means just one of my four cores is maxed out?)

It’s the same issue when running standalone GPU benchmarking apps. I don’t remember it doing this six months ago. I could do stuff in the background with no problem while GPU rendering. Any ideas of what I should look into?