Lazy Trees Land in Determinate Nix 3.5.2: A Massive Performance Boost for Nix
2025-05-26
Determinate Nix 3.5.2 has landed, bringing with it the highly anticipated lazy trees feature. This significantly improves Nix's efficiency in large repositories, especially massive monorepos, by using a virtual filesystem to optimize file copying. Tests show evaluations in Nixpkgs can be 3x faster and use 20x less disk space. Users can opt-in by enabling `lazy-trees = true` in their custom Nix configuration. While currently opt-in, the team has submitted a pull request to merge this into upstream Nix. Future performance improvements are planned, including better evaluation caching, parallel evaluation for more Nix operations, and multi-threaded unpacking of flakes.
Development
Lazy Trees