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Chapter 12. Flushing the Document Early

The Performance Golden Rule reminds us to focus our performance improvements on the frontend—that’s where most of the time is spent loading web pages.[53] Occasionally, there are exceptions to this rule where the backend takes a long time to generate the HTML document. Such a page might require intensive database queries or responses from other web services before the HTML content is returned.

Unfortunately, while the backend chugs away, everything on the user’s end is on hold. Rather than letting the browser sit idle and leaving the user waiting for feedback, this chapter explains how to start the page loading even before the HTML document is completed.

Flush the Head

In most cases, the browser waits for the HTML document to arrive before it starts rendering the page and downloading the page’s resources. This is shown by the Simple Page example.

This example page contains two images and a script. The HTML document and its three resources are all programmed to take two seconds to return. The HTTP waterfall chart for Simple Page is shown in Figure 12-1. As expected, the HTML document is downloaded first. Once it arrives, the browser parses the HTML, renders the first few lines of text, and starts downloading the page’s resources.

Simple Page HTTP waterfall chart

Figure 12-1. Simple Page HTTP waterfall chart

The two images, served from ...

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