The following are the primary take-aways from this "Why so Slow" section of What Makes the Web Work?.
When you go to a web page, a request for some HTML is made. But this is just the start. Included in this HTML will be instructions for numerous additional requests. For example, go to www.amazon.com and you will over 400 requests for additional "assets".
There a many other types of "assets" that web pages use to create the user's experience. Other types of "assets" include images, video, fonts, and data.
It is common for websites to include many "assets" that are never used (or seen) on a web page. For example, have you every been to a website that includes a great deal of photos but you don't scroll down to view them all?
Of course, the fewer the "assets" the quicker the page will load. But once you have determine the assets that you need on a page, there are a number of ways to speed up the delivery of these assets.
CDN(Content Delivery Network) to deliver your assets even faster.