It’s a jungle out there. Optimizing for mobile performance takes a delicate balance of keeping a site lean, fast and displaying as much content—if not all of it—as quickly as possible. And Amazon.com Inc. is proving it’s prowess at surviving and thriving in the mobile site performance forest.

Amazon.com Inc. climbed in the rankings in this week’s AppDynamics/Mobile Strategies 360 mobile index, which ranks 30 businesses’ mobile site performance each week. Meanwhile, J. Crew could use some mobile performance style tips as it fell in the ranking for the week ending Oct. 18.

“The real story for most of this week’s big movers is about changes in the Speed Index, which is the ratio of content measured over time—sites that render more content sooner (on their mobile site home pages) will have a better speed score because an end user will perceive them to be faster,” says Peter Kacandes, senior product marketing manager, mobile, web and synthetics, at AppDynamics.

For example, on 3G networks Amazon moved up this week from No. 8 to No. 6 with an improvement in Speed Score from 6.8 to 6.1 seconds, despite the fact that Amazon had a slightly heavier home page weight (2.49 megabytes for the week ending Oct. 18 compared with 2.38 megabytes a week earlier) and a longer visually complete time, or the time it takes for the browser to finish drawing all the elements of a page (21.5 seconds compared with 19.9 seconds). However, it loaded more content quickly, which improved its overall Speed Score on the index.

J. Crew, meanwhile, fell culprit to a heavy home page weight which resulted in it falling 11 places in 3G to No. 16 and five places in 4G to No. 12. J. Crew more than doubled its home page weight on 3G going from .7 megabytes to 1.53 megabytes. That led to a longer visually complete time (11.1 to 12.8 seconds) and an increase in Speed Score from 6.0 to 8.5 seconds, Kacandes says. This is a major change of pace for the fashion retailer, which rose 11 sports in 3G in the previous week.  J. Crew did not respond to a request to comment on its mobile site performance.

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 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 Oct. 18, the average Speed Score for 3G was 9.3 seconds, the same as a week earlier. The average first render was 6.6 seconds, also the same as a week earlier.

Visually complete average was 15.2 seconds compared with 15.3 seconds the previous week. Average number of elements loaded was 95.4 compared to 95.2 a week earlier. Average home page weight was 1.7 megabytes compared to 1.5 megabytes a week earlier.

On 4G, the average Speed Score was 5.4 seconds compared with 5.2 seconds a week earlier. The average first render was 3.8 seconds the same as the previous week.

Visually complete average time was 8.9 seconds compared with 8.4 seconds the previous week. Average number of elements loaded was 96.7 versus 95.4 a week earlier. And average home page weight was 1.9 megabytes compared to 1.6 megabytes a week earlier.

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 Oct. 18, 2015.

Sign up for a free subscription to Mobile Strategies 360, a newsletter reporting on how businesses in all industries use mobile technologies to communicate with and market and sell to their consumers. Mobile Strategies 360 is published by Vertical Web Media LLC, which also publishes Internet Retailer, a business publication on e-retailing.