This really depends on the settings and the use. If you are using it for widgets or somewhat infrequent updates, CPU usage is below 10% for most cases and 2-3% when idle. During active use, use the setting descriptions below to choose the best option for you.
Regular resolution uses the lowest CPU. It is perfect for most use cases and puts almost no stress on your computer (between 15-25% for active use).
If you either want more space or very sharp retina resolution, you should use retina or high resolution. However, the display has four times the pixels and therefore increases the load on your GPU and CPU by 2-3x.
If you want the maximum quality experience go with 60 FPS. This will require more CPU power but provide an incredibly smooth display. However, if you're on the go and don't mind running at the same frame rate as a television, choose 30 FPS. It uses about half the resources and will help your battery last longer. In cases where you are having trouble running the max quality 60 fps smoothly, 30 fps will give you more responsive performance by lightening the load on your CPU.
The last option will affect image quality. If you are using it on the go and not plugged into an outlet, Energy Efficient will suit you best. If you're plugged in and have a modern Mac, you should go High Power! The image is clearer and you'll get a great experience. High Power mode uses about 10-15% more resources than Energy Efficient mode.