I think it gets to the fundamental problem of what can you do during build-time, and what has to wait during run-time. For example, you may have a CORS plug, but that requires inputs for valid origins. You could pre-populate the list of origins so that it is known at build time, or you could defer that decision to run-time and populate that list with an env var. The latter may be ever so slightly slower, but you also net having your build artifacts be environment agnostic and portable. I def agree that whatever can be determined at build time, should be done when the application is compiled.

As for the performance cost…I think it depends. If you configuration is in an env var or app config, the look up time is negligible. If your config if a network hop away…then yes. At which point you should reach for something like https://hexdocs.pm/elixir/master/Config.Provider.html to resolve the configuration and store it in app config (at run-time).