720P measures only the CPU. It basically eliminates the GPU from the equation. Back in the day when every gamed at 1024x768 and 1600x1200, all of the CPU reviews would use 640x480 or 800x600. However, I think reviews that ONLY do 720p are kinda useless, as at that point you're basically measuring how fast the CPU can execute the game engine's main thread. At higher resolutions, the CPU also starts getting busier doing other things - like issuing draw calls to the GPU, computing geometry, etc... I think these days, 1080p is a good balance between GPU use and CPU usage.