Sometimes a low ranking on the AppDynamics/Mobile Strategies 360 Performance Index doesn’t tell the whole story of a mobile site’s performance.

Fathead.com, an e-retailer of sports and entertainment decals, not only relinquished its No. 1 spot on the 3G index for the week ending Feb. 14, it plunged 24 spots to No. 25. The index monitors the site speed and general performance of 30 mobile sites.

While this appears to look bad from a ranking perspective, as Michael Layne, director of Internet marketing for Fathead admits, he says his customers are doing just fine. In fact, last week Fathead had the best week in 2016 in terms of overall year over year direct revenue, he says. Sales overall were up 19% year over year, and mobile sales were up 65% year over year.

Fathead’s home page weight grew a modest 300 kilobytes, to 3.1 megabytes compared to 2.8 megabytes the previous month. Plus the e-retailer added only three elements to its home page. However, its visually complete time, or the time it takes for the home page to finish drawing the visual elements on the page, increased six seconds, which is the main reason the e-retailer plummeted in the rankings, says Peter Kacandes, senior product marketing manager, mobile, web and synthetics, for AppDynamics.

Layne agrees with this data, as Fathead did add a slide to its home-page carousel. The second slide appears automatically, after a delay of 6 seconds, Layne says, which is the exact amount of time Fathead was penalized for its visually complete time.

“The customer experience is not affected by this delay,” Layne says. “What the consumer sees in the view port is happening as quickly as ever. We are simply providing additional visual elements and calls to action that expose more of our content and product offering at the top of the screen.”

Layne backs this up by saying Fathead’s mobile bounce rate, or the percent of visitors who leave the mobile site after viewing only one page, has not changed. If consumers were experiencing a longer delay for the mobile site to load, the bounce rate most likely would have shot up, he says.

“This is one of those cases where the numbers in the index don’t really tell the story from a consumer perspective,” Layne says.

Taking Fathead’s No. 1 spot on the 3G index is Etsy Inc., an online marketplace that sells handmade crafts and vintage goods. This is the first time that Etsy has clinched the No. 1 spot on the index.

“Etsy shaved almost 200 kilobytes off of their complete load, while maintaining an average of 62 elements loaded,” Kacandes says.

Etsy also improved its visually complete time by 0.7 seconds, to 7.5 seconds, and improved the time the home page starts drawing the visual elements by 0.5 seconds to 3.2 seconds. Etsy did not respond to a request for comment.

The exclusive AppDynamics/Mobile Strategies 360 Index tracks key mobile site metrics across a range of sites to calculate a weekly average Speed Score on both 3G and 4G mobile carrier networks. The index represents a snapshot of mobile site performance across industries, including retailers such as HSN.com, insurance companies such as NorthwesternMutual.com and restaurants such as McDonalds.com.

AppDynamics each week measures performance metrics of both 3G and 4G networks for each of the 30 sites in the index to determine each site’s Speed Score.

They include:

First render: The time, (in seconds) it takes until the web browser starts drawing the visual elements of a home page.

Visually complete: The time (in seconds) it takes until the web browser has completed drawing the visual elements of a home page.

Number of elements loaded: The number of resources loaded by a home page.

Complete load: The size of the home page when all data is loaded (in megabytes).

Using these metrics, the index calculates the Speed Score. The Speed Score is a ratio that rewards businesses for displaying more visual content on the mobile home page earlier rather than later. The earlier a mobile site home page looks complete to a consumer, the better the mobile experience for the visitor, even if a few remaining elements are displayed later. A site with a higher ratio of content displayed earlier will have a better Speed Score than a site with the same visually complete time but a longer first render time.

For example, site A may have a visually complete home page load time of three seconds, loading 90% of its content in the first second and the remaining 10% in seconds two and three. Site B may have the same three-second visually complete time, but present zero content in the first two seconds and 100% in the third second. The sites have identical visually complete times, but Site A will be perceived as much faster, and will earn a better Speed Score as a result.

The index displays the average across the 30 mobile sites for each metric as well as the average Speed Score on both 3G and 4G networks for each mobile site.

For the week ended Feb. 14, the average Speed Score for 3G was 9.8 seconds compared to 9.4 seconds the previous month. The average first render was 6.6 seconds compared to 6.9 seconds the previous month.

Visually complete average was 15.4 seconds compared with 15.6 seconds the previous month. Average number of elements loaded was 96.0 compared with 97.3 the previous month. Average home page weight was 1.8 megabytes, the same as the previous month.

On 4G, the average Speed Score was 6.4 seconds compared to 5.7 seconds the previous month. The average first render was 4.2 seconds compared with 4.0 seconds the previous month.

Visually complete average time was 9.7 seconds compared to 9.2 seconds the previous month. Average number of elements loaded was 96.2 versus 97.6 a month earlier. And average home page weight was 2.0 megabytes compared with 1.8 megabytes the previous month.

The index is accompanied by a chart on MobileStrategies360.com that is refreshed weekly to illustrate how each mobile site performs as well as index averages. Click here to access the performance chart for the week ending Feb. 14.