Why does test suite performance matter? First of all, testing is a part of a developer's feedback loop (see @searls talk) and, secondly, it is a part of a deployment cycle.

It is not customary in Rails to run the full test suite before pushing changes. The railties test suite in particular takes a long time, and takes an especially long time if the source code is mounted in /vagrant as happens in the recommended workflow with the rails-dev-box.As a compromise, test what your code obviously affects, and if the change is not in railties, run the whole test suite of the affected component. If all tests are passing, that's enough to propose your contribution.