Starting from eba783b, the substituter maintains a local cache (by default under /nix/var/nix/substitute-binary/cache) of lookups of binaries.
So if you look for a given /nix/store file name several times in a row, only the first time does it actually connect to hydra.gnu.org to check if it’s there. (Nix’s substituter implements the same strategy, though slightly differently.) Ludo’.
