Im very familiar with Nix or the language, but why would interpreting guile scheme for package management be expensive? What are guix and nix doing that would require evaluating everything lazily for good enough performance?
let chromium = pkgs.chromium; in 1 + 1
In a maximally eager language you'd need to wait for the entirety of Chromium to build before you can find out what 1 + 1 is.