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Yet we see that StackOverflow is fast and scaling really well with demand while Reddit still has load issues after seven years.

Reddit does get about 15x the traffic SO does. But there are too many confounding variables to draw any conclusions about Linux vs. Windows.

Reddit has rather more complex database queries than StackOverflow so that's not really apples to apples

Genuinely curious as to how you came to that conclusion.From the outside looking in, they both look like they would serve similar queries.

On top of that, SO's search feature is (IMHO) loads better than Reddit's.

StackOverflow uses ElasticSearch (open source, Java) for search.