Although, tooling was very heavily Windows based at the time (no Rider); so it is understandable why StackOverflow took the direction they took.
It was an amazing achievement, but never quite got me there.
Source: Was runnin .NET on Mono 7 years ago or so.
That's not a source. This is a source:
http://www.techempower.com/benchmarks/
Yes, it does show Mono's ASP performance is substantially worse than .NET, and probably more-so in 2008. I never said otherwise.
There are a lot of things to take into consideration when selecting a tech stack for a brand new, entirely unproven product. It's certainly on the list, however if performance ranks highly, then I'd be seriously questioning the diligence of whoever made said list. Expenditure (e.g. Windows Server licensing fees) would rank high on the list, however so would developer familiarity with a given stack as well as tooling familiarity.
As I said, I can see why StackOverflow made the decision they made, but I'm doubtful performance was, at that time, a major driving force.