In the "About" section of SO, it says that it is "Fast. Very, very fast."
What is it that makes it so fast? DB? MVC? What?
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In this answer lies everything that StackOverflow is built with. Using the right tools coupled with solid programming is what makes it fast. However, it doesn't hurt that the servers are cooled with liquid cocaine.
Also, check out the data dump schema. A well-designed database and indexes are absolutely paramount to speed.