Sustainable testing

> Doesn't rails actually glob directories on the autoload path

Absolutely correct — for your application files (models, etc). Globbing and adding to the auto-loader isn't free, though.
My point was the number of dependencies Rails loads _besides_ your app, namely the numerous gems and the enormous ActiveSupport.

Just tested this:
- For a vanilla `rails new` (Rails 4) app, `config/enviroment` loads 630+ Ruby files.
- For our monolith, `config/environment` loads 1800+ Ruby files.

(this is _after_ bundler).