How come Hacker News rarely is down although the traffic seems to be huge?
Because the team is absurdly talented. Dan once wrote his own Node debugger for emacs, and some features were way nicer than even Chrome devtools provides today.
https://github.com/gruseom/numen
There are also very few moving parts. In arc you have access to the entire stack from http request to http response, at the raw text level. The goal is simply to generate the text as quickly as possible. And racket turned out to be a good long term bet; people underestimate how optimized it is.