The initial page load of an SPA-based web application can be perceived to be slow. The slowness can result from the time-consuming, initial download of the aggregated JavaScript payload.

The Transmission Control Protocol (TCP) has a slow start mechanism, where data is sent in segments. The JavaScript payload will require multiple round trips between the server and the client before it can be fully delivered to the web browser:

A consequence of this is that users have to wait for the JavaScript payload to be completely fetched before the web page can be fully rendered. It is a common user experience (UX) practice to use a loading indicator (such as a spinning wheel) to let the user know that the user interface is still loading.

Figure 1.7 includes an illustration (on the left) that depicts the loading indicator, and an illustration (on the right) that depicts the layout of the loaded web page. It is important to note that, depending on the SPA implementation, there may be more than one loading indicator spread across the individual sections that make up the web page.

I'm sure that, in your own web browsing travels, you have probably used web applications that have contained these loading spinners. We can agree, from the user's perspective, that ideally we would rather want to see the rendered output instead of the spinning wheel.