Fast specs - Run your specs in less than 1 second

Okay, let me clarify that title first. I, as most of you, have two sets of tests for my Rails application: rspec and cucumber. rspec heavily focusses on testing models and business logic while cucumber focusses on testing the entire application stack and user interaction. The problem is that as your app grows, your test set grows - and so does the time it takes to run those tests.