Parallelize Downloads Across Hostnames (Deprecated)

Hostname parallelization, also referred to as sharding, is the process of distributing web page resources across multiple subdomains. For example, css.examplesite.com for a set of CSS files or js.examplesite.com for a handful of JavaScript files, and the same for images, and so on. The reason behind this is that browsers can handle, on average, between 4-6 concurrent connections per domain. So, by distributing the load across multiple subdomains, browsers can then manage more concurrent connections at one time. So, if you were to imagine having all these resources on the same domain, it would take longer because they would have to wait for a connection to open. Using hostname parallelization allows these resources to download in parallel, hence the name thereby reducing load times. However, one drawback is that because you are splitting resources across different subdomains, it does add additional DNS lookups.

The reason that PageSpeed has deprecated this optimization is due to the shift from HTTP/1.1 to HTTP/2. When HTTP/2 came along, it improved upon how HTTP/1.1 prioritized content, making it much faster and more efficient. HTTP/1.1 is still used today, but as HTTP/2 grew, there was no longer a need for this optimization. So, the recommendation is to try to reduce or combine your CSS and JavaScript files externally or use image and CSS sprites. Or discontinue using HTTP/1.1 altogether and upgrade your web server to HTTP/2.