What follows is an examination of 10 top U.S. web sites using the rules and tools described in this book. This analysis gives examples of how to identify performance improvements in real-world web sites. My hope is that after walking through these examples you will look at web sites with the critical eye of a performance advocate.
Table 17-1 shows the page weight, response time, and YSlow grade for the home pages of 10 top U.S. web sites as measured in early 2007. YSlow is a performance tool developed at Yahoo! that produces a single score (A is best, F is worst) for how well a page satisfies the performance rules described in this book. See the upcoming section "How the Tests Were Done" for more information.
Not surprisingly, page weight and response time are strongly correlated, with a correlation coefficient of 0.94, as shown in Figure 17-1. This makes sense—adding more components or larger components to the page makes it slower. Plotting page weight and response time throughout the development process is a worthwhile analysis for any web page undergoing performance improvements.