Building a declared environment takes quite a while and switching a NixOS or home-manager generation takes even longer. It was my main complaint about NixOS when I was starting out.

The thing I realised though is that the main scenario where you actually need the speed is when you want to try things out and for that, Nix' ad-hoc environments are much better suited.

When I want to try a new program (btop for example), I simply run `nix-shell -p btop` rather than adding it to my declared environment and sitting through a `nixos-rebuild test`. That's usually faster and, more importantly, cleaner than installing the package to a global environment with a regular package manager.