In this presentation, James Cox shows a bunch of things to find bottlenecks that can bring your down. Hint: the speed of Ruby's interpreter will be very, very far down on your check list.
Architectural decisions such as caching strategies (or caching at all) can make all the difference. The database is also a regular source of performance problems. James shows in which situations it's advisable to optimize or even bypass ActiveRecord.
Deployment is also a topic and a source of performance issues is the choice of a hosting service. James explains why shared hosting services should be avoided, and gives recommendations for providers.
If you are working on a Rails application - this is a must-see. Even if your application doesn't show performance issues yet, it can help you avoid some of the more glaring performance issues. And even if you're not working on Rails - this presentation will still give you lots of ideas and tips for your web application.
Watch "Managing a high performance rails app without tearing your hair out"