Your web browser is pretty smart. It automatically makes as many connections as possible to the host to try and download content in parallel. Connections to a single host are limited, however. When a browser reaches the threshold, it puts other content in a queue. The content in the queue waits until other requests complete, and continues when more connections can be made. The time spent waiting for requests to finish is referred to as blocking. Our goal should be to reduce blocking as much as possible to achieve shorter load times.
One way by which you can improve parallel downloads is by using subdomains; for example,
yoursite.com could have
static-img.yoursite.com. We call this domain sharing. Because you make more hosts available, more connections can be created and more content can be downloaded simultaneously. Your load times can dramatically improve because of this.