Chapter 5 describes how external scripts block parallel downloads and
rendering. It’s not surprising that inline scripts have the same behavior
for the same reasons (preserving execution order and
document.write dependencies). The Inline Scripts
Block example demonstrates this
Figure 6-1 shows the HTTP requests issued for this page. In addition to the HTML document, there are two image requests, each configured to take one second. An inline script is inserted between these two images. The inline script is represented by a line in Figure 6-1. It does not generate an HTTP request, but the impact it has is observable.
The inline script is configured to take five seconds to execute. This is what causes the four seconds ...