Rule number one for high traffic sites with a lot of pageloads coming from search engines: Aggressively cache content shown to unauthenticated users. Since they don't really interact with the site, they probably won't even notice if it's a bit stale.

They used to have a really good blog post about their caching infra which for some reason was deleted. Archive link: https://web.archive.org/web/20210205121832/https://redditblo...