They are on Rails 3 which is pretty old for Rails standards and they got there recently. See http://shayfrendt.com/posts/upgrading-github-to-rails-3-with...

This is their architecture in 2009 https://github.com/blog/530-how-we-made-github-fast Couldn't find anything more recent.

They do, but a real ruby on rails site typically serves most html content out of memcache, even higher for users that aren't logged into the platform. Its not unknown to see 5-10ms response times in those scenarios