Serving resources from four or five different hostnames increases parallelization of downloads.
Page Speed recommends:
Balance parallelizable resources across hostnames
Requests for most static resources, including images, CSS, and other binary objects, can be parallelized. Balance requests to all these objects as much as possible across the hostnames. If that's not possible, as a rule of thumb, try to ensure that no one host serves more than 50% more than the average across all hosts. So, for example, if you have 40 resources, and 4 hosts, each host should serve ideally 10 resources; in the worst case, no host should serve more than 15. If you have 100 resources and 4 hosts, each host should serve 25 resources; no one host should serve more than 38.
On the other hand, many browsers do not download JavaScript files in parallel*, so there is no benefit from serving them from multiple hostnames. So when balancing resources across hostnames, remove any JS files from your allocation equation.
*For a list of browsers that do and do not support parallel downloading of JavaScript files, see Browserscope.
Prevent external JS from blocking parallel downloads
When downloading external JavaScript, many browsers block downloads of all other types of files on all hostnames, regardless of the number of hostnames involved. To prevent JS downloads from blocking other downloads (and to speed up the JS downloads themselves):
Always serve a resource from the same hostname.
To improve the browser cache hit rate, the client should always fetch a resource from the same hostname. Make sure all page references to the same resource use the same URL.
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