While technically possible, you wouldn’t want to compile everything locally on NixOS. Only packages that you’ve made changes to (such as applied a patch) will be built locally, and everything else (by default) will be pulled from the precomputed binary cache.

You can disable the binary cache, or make changes to every package. The thing is, if you update a nix package, you’ll have to rebuild everything that depends on it, and with lower-level components, that can be literally everything. It’s not a sustainable workflow.

NixOS is not the most efficient distro either. I already mentioned some compiler optimizations are disabled by default, because they break build reproducibility. It also tends to use more disk space than other distros. So actually trying to super-optimize every package on it is somewhat pointless.