That plug is not happy. Look at all the sharp edges. They are rounded off. That's a short amount of time on the plugs.
Looking at the square edge on the ground wire you can see the rolled edges. Same with the center wire. NGK plugs don't wear as quick as a Champion but they don't last as long as an Autolite plug.
If you compare a plug out of the box to yours you can see how much the edges are rolled.
Is your timing locked out? If so, you may be putting too much heat in the plug at low speeds.
I'd be looking at doing something to get some heat out of the plug.