Push C. You've got a cordon going on in a large circle. I believe there is a tiny button on the toolbar next to these


but we don't have an image for it. Click it or hitting C toggles it.
A cordon lets you compile a small area of the map without doing the entire thing. Usually it's square, but that one was added recently. It centers on where your camera is, and is probably making your Hammer feel laggy when you fly around the 3D pane.