Currently flakes are evaluated from the Nix store, so when using a local flake, it's first copied to the store. This means that

$ cd /path/to/nixpkgs
$ nix build .#hello

is a lot slower than the non-flake alternative

$ nix build -f . hello

Ideally, we would copy the flake to the store only when its outPath attribute is evaluated. However, we also need to ensure that it's not possible to access untracked files (i.e. we need to check every file against git ls-files).