What yen costs

yen is one file of Nix builtins. Its own cost is small next to nixpkgs and the module system, and it stays small as a fleet grows:

• Each scope computes each aspect ref once, so aspects that read aspects cost linear time, however deep they nest.
• Sinks are lazy per receiver. A sink that nobody reads costs nothing.
• Declarations are one lazy index over every file: a lookup walks its path, never all files.

Cost oracle (tests/cost.sh)	Bound	Without the memo
30 levels of aspect-ref diamonds	under 200,000 function calls	26 million calls at 16 levels
a 150-host fan-in	under 200,000 function calls	954,000 calls

A memo is invisible to an eval, so the oracle bounds Nix’s own counters (NIX_SHOW_STATS), not wall time. The engine evaluates 10,000 modules in about 20 s.

nix-instantiate --eval --strict tests -A failed   # [ ]: the engine suites
bash tests/cost.sh                                # PASS aspect-ref-memo, PASS reduction-memo

Where the time goes

On a real host, nixpkgs and the NixOS module system are most of the evaluation. See Shared pkgs and Trimmed module lists.