I am wondering why/how Stack Overflow is so fast.
"Stack Overflow is a programming Q & A site that's free. ... And fast. Very, very fast."
-the about page
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I am wondering why Stackoverflow is so fast?
Stack Overflow was made to be fast because a quick and responsive browsing experience really improves the usability of the site.
In general, faster is better. That's why it is optimized for speed.
As for how, cache, cache, cache. Just about every page you will normally hit is a cached version. This really reduces load on the servers. They also do things like using one image for most of the small icons. This reduces the number of resources required to download.
And, they have a special cookie-free domain (sstatic.net) for hosting their static content. That really speeds up the static content.
I interpreted the phrase you quote in your question as meaning that you will get answers to your questions fast not that the site itself is fast.
The median response time between a question being posted and its subsequently accepted answer is 21 minutes.
The reputation system and voting patterns tend to incentivise fast answers.