Google Maps, Yahoo! Mail, Facebook, MySpace, YouTube, and Amazon are examples of Web sites built to scale. They access petabytes of data sending terabits per second to millions of users worldwide. The magnitude is awe-inspiring. Users view these large-scale Web sites from a narrower perspective. The typical user has megabytes of data that are downloaded at a few hundred kilobits per second. Users are not so interested in the massive number of requests per second being served; they care more about their individual requests. As they use these Web applications, they inevitably ask the same question: "Why is this site so slow?" The answer hinges on where development teams focus their performance improvements. Performance for the sake of scalability is rightly focused on the back end. Database tuning, replicating architectures, customized data caching, and so on allow Web servers to handle a greater number of requests.