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We had a question in class about why turning on supersampling (e.g., 8x, which should be taking 8 times the number of samples) does not tend to cause a corresponding decrease in runtime. The short answer is that there is some cleverness going on under the hood that cuts down on the number of samples that have to be taken in total to get each individual frame on the screen. The following is one reference for how some of this is done and includes a small set of timing numbers:

From the posting, we also see that MLAA (Morphological Anti-aliasing) is a postprocessing step on the entire image, and so it will not have the same scaling behavior, because it does not require taking more samples. Instead, it looks for edges in the image and based on the edge geometry applies an appropriate blur using image processing techniques to make them look better.