High Performance Web Sites

Where Does the Time Go?

Looking at the HTTP traffic in this way, we see that at least 80% of the end user response time is spent on the components in the page. If we dig deeper into the details of these charts, we start to see how complex the interplay between browsers and HTTP becomes. Earlier, I mentioned how the HTTP status codes and headers affect the browser's cache. In addition, we can make these observations:

• The cached scenario (Figure 1-2) doesn't have as much download activity. Instead, you can see a blank space with no downloads that occurs immediately following the HTML document's HTTP request. This is time when the browser is parsing HTML, JavaScript, and CSS, and retrieving components from its cache.

• Varying numbers of HTTP requests occur in parallel. Figure 1-2 has a maximum of three HTTP requests happening in parallel, whereas in Figure 1-1, there are as many as six or seven simultaneous HTTP requests. This behavior is due to the number of different hostnames being used, and whether they use HTTP/1.0 or HTTP/1.1. Chapter 8 explains these issues in the section "Parallel Downloads."

• Parallel requests don't happen during requests for scripts. That's because in most situations, browsers block additional HTTP requests while they download scripts. See Chapter 8 to understand why this happens and how to use this knowledge to improve page load times.

Figuring out exactly where the time goes is a challenge. But it's easy to see where the time does not go—it does not go into downloading the HTML document, including any backend processing. That's why frontend performance is important.