Page Speed: Minimize DNS lookups

Page Speed recommends:

Use URL paths instead of hostnames wherever possible

If you host multiple properties on the same domain, assign those properties to URL paths rather than separate hostnames. For example, instead of hosting your developer site on developer.example.com, host it on www.example.com/developer. Unless there are good technical reasons to use different hostnames, e.g. to implement DNS-based traffic load-balancing policies, there's no advantage in using a hostname over a URL path to encode a product name. In fact, the latency improvements of consolidating properties on a single hostname can actually enhance user experience: users can link between properties in a single browsing session with no additional DNS lookup penalty. In addition, reusing domains allows the browser to reuse TCP connections more frequently, further reducing the number of round trip times. If one property receives a lot of traffic, reusing that hostname for other properties can also increase the DNS cache hit rate for first-time visits, since the likelihood of a valid mapping already existing on a local caching server is higher.

Serve early-loaded JavaScript files from the same hostname as the main document.

It's especially important to minimize lookups in the "critical path". We define the critical path as the code and resources required to render the initial view of a web page. In particular, external JavaScript files that you own and that are loaded from the document head, or early in the document body, should be served from the same host as the main document. Most browsers block other downloads and rendering while JavaScript files are being downloaded, parsed and executed. Adding DNS lookup time to this process further delays page load time. If it's not possible to serve those files from the same hostname as the containing document, defer loading them, if possible. The exception is for JS files that are shared by pages served off multiple domains: in this case, serving the file from a unique URL to increase the cache hit rate may outweigh the DNS lookup overhead.

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