HTTP uses headers to provide metadata about a request along with the main payload in the body of the message, much like emails do. You won't see these headers when you view the source, but you can observe them using the browser developer tools. You can use headers for many things, such as cache control and authentication. Cookies are also sent and received as headers.
Browsers will only open a limited number of HTTP/1.1 connections at one time to a single host. If you require a lot of requests to retrieve all the assets for a page, then they are queued, which increases the total time taken to fully load it. When combined with the TCP slow-start mentioned previously, this effect can be amplified, degrading the performance. This is ...
