They didn't start entirely from scratch, they've been doing this with PHP already: > Internally, Pyre's high-level architecture is similar to that of Hack, Fac

They didn't start entirely from scratch, they've been doing this with PHP already: > Internally, Pyre's high-level architecture is similar to that of Hack, Facebook's type checker for PHP. If they had a performant codebase to start with, this makes a lot of sense.