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
Tools to measure:
jMeter Rails guides for performance testing
Gems that will help you detect problems
Query Reviewer Rails Footnotes New Relic Perftools
The xkcd comic that inspired the test example. The subscription link to Ben Orenstein’s emails with suggestions to speed up the test cycle.
Measure, know where to look and most important write enough unit/functional tests to avoid breaking havoc on your app!