When testing a website for performance using popular online tools like GTMetrix, Pingdom, Google PageSpeed, sometime we might get “Parallelize Downloads Across Hostnames” warning. This warning means that your website’s assets are delivered from the same host name, and it would slower the site’s speed.
There is a limitation on the number of concurrent connection that web browsers will make to a host. For HTTP/1 and HTTP/1.1 connections, a web browser is only able to download one file at a time.