A mild breeze shook up the mobile site rankings in the AppDynamics/Mobile Strategies 360 Index for the week ending April 17.

While some mobile sites stayed rooted in their positions—like No. 1 Esty.com and No. 2 Gilt.com on 3G—other sites slightly moved up or down the 3G index, such as Costco.com and TheCheeseCakeFactory.com, which swapped their No. 3 and No. 4 spots.

“This week’s results shows that while there was a lot of stability in 3G, there was also some movement, though mostly just a few spots up or down,” says Peter Kacandes, senior product marketing manager, mobile, web and synthetics, for AppDynamics.

Amazon.com’s mobile site again moved up in the rankings on 3G to No. 5 from No. 7 the previous week. Amazon reduced its mobile site home page weight by 360 kilobytes, which sliced two seconds off the visually complete time, or the time it takes for the browser to finish drawing the visual elements on the page, Kacandes says.

And despite Morton’s maintaining its mobile performance numbers, Amazon’s upward movement bumped Mortons.com down one spot to No. 6. Morton’s did not respond to a request for comment.

On 4G, Amazon jumped up five spots to No. 3 from No. 8 after decreasing the home page weight and decreasing its visually complete time to 9.0 seconds from 10.7 seconds. Mortons.com, however, maintained its No. 1 spot on 4G. Amazon didn’t respond to a request for comment.

Travelocity.com’s mobile site also maintained consistent mobile performance numbers week to week, but moved up two spots on the 3G index to No. 7 from No. 9.

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 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 April 17.