Yes it by default build everything from source. However, most of the packages will have binary cache on https://cache.nixos.org/ https://cache.nixos.org/ so tha

Yes it by default build everything from source. However, most of the packages will have binary cache on https://cache.nixos.org/ https://cache.nixos.org/ so that your installation or update will download them instead of building them. Also you can setup your own binary cache (https://nixos.wiki/wiki/Binary_Cache https://nixos.wiki/wiki/Binary_Cache) and building machine to make your own projects build faster.