There comes a point in the life of most applications when the people using it complain about it. Sometimes this is down to the usability of the application's front end—buttons in the wrong place, tortuous workflow, bad color choices, small fonts, etc. This is largely down to interface design, an enormous topic outside the scope of this book.
Other times, an application may have a great interface but still be unusable. Often, this is because it's just too slow. In the case of Rails, this problem might arise sooner than you expect. The Ruby interpreters available at present (mid 2007) are quite slow themselves; coupled with that, all the clever meta-programming that makes Rails such a pleasure for developers turns it into a resource-hogging nightmare for system administrators.
Slowness is something you can deal with, requiring minimal artistry and resources. This section covers how to track down particular issues with your application, and what to do about them...