Another reason for slow compilation is if given module depends on other slow modules. [1] Is a great resource for understanding compilation. In general, you want to avoid compile-time dependencies between modules as much as possible.

On our Phoenix app, switching from import MyApp.Router.Helpers to alias MyApp.Router.Helpers, as: Routes speed up compilation and limited re-compilations greatly, see: [2]

Elixir 1.6 will ship with improvements to mix xref which should make it easier to investigate this stuff.

[1] http://milhouseonsoftware.com/2016/08/11/understanding-elixir-recompilation/
[2] Do not set compile time dependencies on the router · Issue #2530 · phoenixframework/phoenix · GitHub
[3] elixir/CHANGELOG.md at main · elixir-lang/elixir · GitHub

A possible partial fix

instead of using multiple function heads

def fun(a, :foo = b), do: :alpha 
def fun(a, :bar = b), do: :beta

you can help the compiler by making it one function head with embedded case statement:

def fun (a, b) do
  case b do
   :foo -> :alpha 
   :bar -> :beta
end

Routex was not able to build helper functions for 400 (generated) routes causing “too complex” issues during compilation due to the amount of generated function heads. In the upcoming version 1.2 those are rewritten to case statement variants and it now handles more than 2.000 routes.

Yes. I do not expect this for manually written code. If you write 400 function heads yourself, you might be the limit yourself

Fully agree with your advice