Then why would a build take so long as GP describes?

I can't imagine why some unknown people encountered difficulty building some unknown project.

We use Nix to build and deploy our big Haskell monolith along with the operating system itself, and all system dependencies, e.g., PostgreSQL, Redis, Grafana, collectd, influxdb, openssh, ejabberd, Elm applications, etc, and it works fabulously.

GP here. My guess is that they intentionally disabled binary caching because this is a financial application and they wanted to make sure no untrusted binaries got into the build pipeline.

Thanks! That makes some sense. Though, I work for a financial service provider as well and have never found compile-from source to be solution to that problem. If you do have dependencies, whether you trust the sources or the binaries shouldn't really matter, as long as you trust the repositories where they come from. And if you don't trust it I wanna see the army of developers actually checking all the sources down to the system level.