When I run `nixos-rebuild switch --verbose`, it does seem to go through the entirety of nixpkgs. Especially noticeable on a RPi with its slow I/O.

It's not true that it pulls in everything. It's just that a nixos installation pulls in a lot of transitive code =). It actually pulls all of this in lazily and lazily evaluates it; but it's just a lot of code and nix evaluator isn't that fast.

That’s just evaluation. The next bigger change in nix itself will likely be the ability to better cache evaluations, significantly speeding up this.