How to make Nix five times faster

I have a confession: I love Nix. So much so, that I want to use it to build all my software. But there's a catch - why is Nix so slow? My search for answers began with a bash loop run amok, and ends with Fantail - a new frontend development server powered by Nix and Haskell, with builds 5x faster than the Nix CLI. If you've ever looked up at the sky and wondered if it's possible to use the Nix C++ interface to cache Flake inputs in-memory, this is the talk for you. If not, come along anyway!