Background

I am reading “Functional Web Development with Elixir, OTP and Phoenix” and in the book the author makes a surprising statement:

(…) In order to shorten that runtime check on an %Island{} struct, let’s add an alias at the top of the module file:
alias IslandsEngine.Island
(…)

WHat?

Perhaps I am miss interpreting this, but what I understand is that by adding the alias of a struct to a module, we actually improve runtime speed on checks that use that struct.

I find this makes no sense at all. Using an alias only affects how the file is compiled, not how fast it runs on my machine.
Can someone explain this to me?