A sample (and reprehensible) application that demonstrates built-in Rails profiling and benchmarking tests.
bundle install
bundle exec rake db:migrate
FauxFile uses ruby-prof, which may have build dependencies; I'm not really sure.
Profile the test suite
rake test:profile
Then take a peek at tmp/performance for a gazillion files with detailed profiles.
I'm no expert. ruby-prof.rubyforge.org has lots of information.