10 minutes is still an order of magnitude slower than a nixos-rebuild if you're hitting the cache, and there's no issue with forgetting to rebuild the guix profile. If you include nonguix (as is required for the 99% of users who need nonfree blobs) you get the 30-50 minutes as described in the article. I believe Nix-the-language being lazily evaluated is a big part of this; you can even inadvertently evaluate nixpkgs multiple times and not notice.