High CPU usage when moving fences around

I decided to re-organize my desktop a little, and was moving some fences around. Since I like my fences neat and evenly spaced, I was holding down shift to have more control over where they were placed. While holding shift I clicked and started to drag the first fence. Within a few seconds, the CPU usage of one of the cores jumped to 89-99% and dragging became extremely laggy and staggered.

I'm on an I7 machine running Windows 7 64bit with 12gb RAM