I have a flake-parts + crane flake package that I build as part of an integration workflow. I have noticed that after upgrading to nix v2.21 building a derivation that should be cached takes 30+ seconds, whereas it is almost instantaneous in v2.18. See the difference in evaluation logs here for v2.18 nix2.18_build.log · GitHub and here for v2.21 nix2.21_build.log · GitHub