Because the light from the center strikes our eyes first, that is the reflection that we see. The variations in speed at the ends is greater, so the hand on the stopwatch would show a greater variation. We don t see this light, because the photons tend to cancel each other out. The hand of our imaginary stopwatch would look something like the following for various spots on the mirror. It is the speed of different colored light that is most important, and as a result, we're able to see the rainbow effect when we look at a diffraction grating.
Light passing through a lens takes advantage of the varying speed through the medium.
It is the study of quantum electrodynamics that addresses the questions about light. This is pretty deep science. I've tried here to describe some of the things that have been said about light, with the help of admittedly over-simplified illustrations. The beauty of this work is that not only did he use very few formulas, but he presented the material in a conversational style. The book requires careful reading, but the material becomes understandable as you follow his line of thought and perhaps review certain passages.