Some web pages have all their HTTP requests served from one domain.
Other sites spread their resources across multiple domains. Rule 9 from
High Performance Web
Sites says to reduce DNS lookups, but sometimes
increasing the number of domains is better for performance, even at the cost
of adding more DNS lookups. The key is to find the web page’s critical path.
If the critical path results from too many resources being served from one
domain, splitting them across multiple domains—what I call domain
sharding—may make the page load more quickly.
Figure 11-1 shows the HTTP
profile for eBay. The horizontal axis represents response time. A steep slope,
as shown on the righthand side of the chart, reflects a lot of downloads
in a short period of time. This is a sign of a fast page. In contrast, a
flat slope such as the one shown in the first five HTTP requests means the
In this case, eBay’s critical path is blocked by the HTML document in the
fourth and fifth requests.
Yahoo!’s HTTP profile, shown in Figure 11-2, has a different critical
path. The majority of the time loading this page is spent downloading
images two at a time. All of the resources in the page are downloaded from a
single domain: l.yimg.com. Some browsers, ...
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