Maybe I missed it, but he did not seem to go into detail as to how the pixel was plotted. If the pixel was placed in a buffer, and then a texture was locked and the pixel was transfered, this would obviously induce significant overhead on its own (any time you need to communicate between CPU/GPU memory induces a huge lag, in shorter words)...although Carmack knows this better than anyone else.

Also, the actual pressing of the button could use more clarification. When did measurement start? On the button down signal? On the button up signal? As soon as the finger touches the button?