Comment by throwawayqqq11

You are describing a propper dependency/code hierarchy.

The merging of attribute sets/modules into a full NixosConfiguration makes this easy. You have one company/product wide module with a bunch stuff in it and many specialized modules with small individual settings for e.g. customers.

Sure, building a complete binary/service/container/nixos can still take plenty of time but if this is your only target to test with, you'd have that effort with any naive build system. But nix isnt one of them.

I think that's the real issue here. Modularizing your software/systems and testing modules as independently as possible. You could write test nix modules with a bunch of assertions and have it evaluate at build time. You could build a foundation service and hot plug different configurations/data, build with nix, into it for testing. You could make test results nix derivations so they dont get rerun when nothing changed.

Nix is slow, yes. But only if you dont structure your code in a way to tame all that redundant work, it comes around and bites you. Consider how slow eg. make is and much its not a big issue for make.