No less than 60-95% of the response "time is spent making HTTP requests to fetch all the components in that HTML document (i.e. images, scripts, and stylesheets)."
The impact of having many components in the page is exacerbated by the fact that browsers download only two or four components in parallel per hostname, depending on the HTTP version of the response and the user’s browser. Our experience shows that reducing the number of HTTP requests has the biggest impact on reducing response time and is often the easiest performance improvement to make.
If a web page evenly distributed its components across two hostnames, the overall response time would be about twice as fast.
Performance Research, Part 4: Maximizing Parallel Downloads in the Carpool Lane