Question about aspect ratio and performance

It is going to do one of two things to the image, it will either stretch and distort causing it to look strange, or it will letter box it by putting a black bar at the top and bottom.

By reducing the resolution you have reduced the number of pixels the GPU needs to render, when letter boxed it would increase your frame rate, if it stretches it i am not sure what the end result will be as im not sure if the monitor or the GPU handles that.

In general fewer pixels means less load on the GPU, but it is usually best to stick with your native resolution or at least the same aspect ratio just to avoid distortion

Letterbox means it has increased FPS then?