Consumers are increasingly impatient when it comes to waiting for their web browsers to load content. Measuring and monitoring how long it takes pages on your site to load — from desktop computers, laptops, tablets, or smartphones — enable you to take corrective action when any one of dozens of site-slowing scenarios occurs.

The most cited recent statistic related to load times for ecommerce sites comes a study that Forrester conducted in 2009 on behalf of Akamai. It read, in part, “40 percent of shoppers will wait no more than three seconds before abandoning a retail or travel site.”

From that study, and from commonsense observations, we all know that load times affect a site's revenue-generating potential. In this article, I'll explain the factors that affect load times, and how to measure and monitor it all.

Drivers of Site Performance

From a user’s perspective, a “page” is the URL in his or her browser. But, the reality is much more complicated. The browser makes dozens of behind-the-scenes calls when a single page loads. These include the following.

• Many parts to a web page. Multiple calls go to your web server to load the base HTML for the page, stylesheet(s) needed, JavaScript files used on your site, and each of the images used to build the page.
• Incorporating social media links. Calls go to each of the different social networks that you have integrated with your site to enable users to easily share content.
• Links to analytics platforms. Calls go to your web analytics platform to capture and record the page view itself.
• Links to ad servers. Calls go to ad servers if you are running digital media campaigns to record the results of your ads and to enable retargeting of ads.

To build a page, it is not uncommon for a browser to make one or more calls to 20 or more different “sites.” Depending on how the page is built, a glitch or delay in any of those sites can hold up the loading of the entire page. Furthermore, these glitches may be isolated to specific groups of visitors: those in a certain country or region or using a certain browser, for instance.

There are several different ways to track how quickly and reliably the pages on your site load. None of them are perfect, and none of them are bulletproof. The best strategy is to take a multi-pronged approach.

Know What Loads

While it’s not on-going measurement or monitoring, simply getting familiar with all of the different components that load on your site is a good place to start. The Firebug plugin for Firefox is a free way to get a visual display of what your browser is actually doing behind the scenes to render a page on your site.

Below is a screenshot from Firebug of an article on this site. In this case, there were 94 individual requests made by the browser to 12 different domains.

Enlarge This Image

Scrolling through the Firebug screen shows:

• Each individual image, JavaScript file, and CSS file that loads;
• How long it takes from the initiation of the request by your browser to the asset being returned and loaded;
• The “Status” of each request — see a number of “304 Not Modified” entries when you load a page on your site, because you visit your site often and, therefore, have a number of assets cached by your browers. Hold down SHIFT and refresh the page, and you will not see any 304 codes, but, instead, “200 OK” codes. This is a closer approximation of how the page loads for first-time visitors to your site.

Keep in mind that this inspection is only one user — i.e., you — loading one page at one point in time from one geographic location. It is a good way to get an understanding of the individual elements of the pages on your site, as well as to uncover major errors with the site. But that’s where the value ends.