Autotest relies on filesystem polling to detect modifications in source code files. This is expensive for the CPU, harddrive and battery - and unnecesary on Mac OS X 10.5 or higher which comes with the very efficient FSEvent core service for this very purpose. This gem teaches autotest how to use FSEvent.
Gemfile gem 'autotest-fsevent', '~> 0.2.3'
install gem install autotest-fsevent -v 0.2.3