Even Faster Web Sites

Chapter 3. Splitting the Initial Payload

The growing adoption of Ajax and DHTML (Dynamic HTML) means today’s web pages have more JavaScript and CSS than ever before. Web applications are becoming more like desktop applications, and just like desktop applications, a large percentage of the application code isn’t used at startup. Advanced desktop applications have a plug-in architecture that allows for modules to be loaded dynamically, a practice that many Web 2.0 applications could benefit from. In this chapter I show some popular Web 2.0 applications that load too much code upfront, and I discuss approaches for making pages load more dynamically.

Kitchen Sink

Facebook has 14 external scripts totaling 786 KB uncompressed.^[5] Figuring out how much of that JavaScript is necessary for the initial page to render is difficult to do, even for a core Facebook frontend engineer. Some of those 14 external scripts are critical to rendering the initial page, but others were included because they support Ajax and DHTML functionality, such as the drop-down menus and the Comment and Like features shown in Figure 3-1.

It’s critical to render a web page as quickly as possible. Doing so engages the user and creates a responsive experience for her. Imagine if the Facebook JavaScript could be split into two parts: what’s needed to render the initial page, and everything else. Rather than bog down the user’s first impression with “everything else,” the initial JavaScript download should ...