I read an article about StackExchange's servers and it seemed super optimized, although I'm not an expert. I can't imagine AWS would be as optimized for exactly StackExchange's requirements. So I think StackExchange would run slower on AWS.
Reddit, on the other hand, is the worst performing out of all the top sites that I can think of.
Reddit, on the other hand, is the worst performing out of all the top sites that I can think of.