Load times among America’s top retail sites have increased by 22% in just one year from an average 5.94 seconds in December 2011 to 7.25 seconds in December 2012.

The findings, which come from a report by Radware, are bad news for ecommerce sites considering the importance of site speed for traffic and conversions.

A previous survey from Tagman found that a one second delay in page-load can cause 7% loss in customer conversions, while a separate survey from Akamai revealed that 40% of people will abandon a web page if it takes more than three seconds to load.

To find out how the top sites were performing Radware tested the load times (in Internet Explorer 9, Firefox 17, and Chrome 23) and page composition of the 2,000 top US retail web sites in December 2012. 

It then compared this data versus previous benchmark tests performed on the same set of sites, dating back to December 2010.

Though overall the average site speed has got slower, the top performing sites have been getting quicker since 2011.

In December 2011 Nike.com clocked the fastest load time at 2.27 seconds, but in 2012 CVS.com topped the list with a load time of just 1.02 seconds.

What’s causing the slowdown?

Between 2011 and 2012 the median number of requests (such as images, HTML, and CSS/JavaScript files) increased 8.22% from 73 to 79.

Each page resource makes an individual round trip from the user’s browser, which requests the file from the host server, which in turn delivers the file to the browser.

Each round trip can take 20-50 milliseconds for desktop browsers – and up to a full second each for mobile users – numbers that add up quickly when pages contain dozens of resources.

Top 100 are slower than average

The data shows that the top 100 sites are actually slower than average with a median load time of 8.23 seconds, 14% slower than the overall load time of 7.25 seconds.

Top retailers are also slowing down at a faster rate than the top 2,000 sites. In the past year the load time for the median top 100 site has slumped from 6.4 seconds to 8.23 seconds, which represents a slowdown of 28% compared to 22% for the top 2,000.

The report suggests that page size and complexity could be partially to blame, as top pages are likely to contain more page resources than other sites.