Here are the slides from my talk on Performance at http://rubyconfuruguay.org
Below you can find the mentioned references and tools.
Why does performance matter?
Google Search uses site speed in search Ranking Marissa Mayer at web 2.0 The Google Gospel of Speed Nielsen Norman group on web response times Articulo pdf de la Unversidad de Nebraska sobre tolerancia de dos segundos
Tools to measure:
Gems that will help you detect problems
Measure, know where to look and most important write enough unit/functional tests to avoid breaking havoc on your app!