Keynote Systems today released the May results of its Mobile News and Portal Index, exclusive to Wireless Week, along with analysis of how breaking news of the assassination of Osama bin Laden affected mobile websites.

As seen in April, Facebook and Google continued to be the top sites for page load time in May. Facebook averaged a page load time of 3.86 seconds, while Google trailed by 0.74 seconds with a 4.6 seconds aggregate time.

Overall average page load time of all 14 sites on the News and Portal index was 13.08 seconds. When it comes to overall mobile website availability, The Weather Channel delivered the highest success rate (availability) with 99.38 percent; it gained three places in the index compared to April availability and showed significant improvement since March when overall availability was 96.01 percent and ranked 13 out of 14 sites.

The Weather Channel knocked down Yahoo in May by one place to second with 99.3 percent success rate. The overall average availability of all 14 sites was 98.23 percent.

In May, Keynote took a look at one news agency’s mobile website, which it refrained from naming, to understand why it suffered a major decline in performance.

At the beginning of May, the news organization’s mobile website page load time was 15.22 seconds with an average object count of 37.4 and took 87.5 Kbytes to download the page. Around mid-May, the site’s page load time increased to 18.92 seconds with an average object count of 54.2 and took 156.9 Kbytes to download the page, the page load time became 3.7 seconds slower and the page weight nearly doubled.

Several days later, the page weight of this news mobile site increased again, and as a result, the page load time increased to 23.8 seconds with an average object count of 75.7 and took 420.7 Kbytes to download the page.

At its peak, the page weight increased nearly five times compared with what it was at the beginning of the month. By the end of May, the page object count and page weight returned to approximately the same level as around mid-May. But these page changes had already impacted end user experience; there were five days where the page took longer than 28 seconds to download over the Sprint network, which is slower than the slowest site average on the index.

The nature of News, Portal and Media mobile sites are to provide end users with the latest news and information and the content of these sites is highly dynamic and constantly being updated. Keynote says it is because of this reason that rapid site changes could easily degrade a site’s performance, especially the performance for mobile end users accessing the site over wireless carrier networks, which typically exhibit lower bandwidth and higher latency than connected websites.

But it wasn’t just page modifications that caused trouble for mobile websites in May. According to Keynote, news of Osama bin Laden’s death triggered sudden increases in both Web and mobile traffic, as many users sought the latest information on the story. As the story broke, news sites began to slow or become unresponsive.

Keynote found that one “leading international news organization” redirected mobile users to its Web page, which proved a good interim solution, effectively redirecting a high volume of mobile user traffic to their website, so that mobile users could at least load the Web page.

However, the company notes that the Web page was not optimized for mobile devices and consequently took longer to load.

Keynote repeatedly tests the sites in its index hourly and around the clock from four locations over the four largest U.S. wireless networks, emulating the browsers of four different devices:  the iPhone 4 on AT&T, the HTC Evo (Android operating system) on Sprint, the Motorola Droid X (Android OS) on Verizon Wireless and the BlackBerry Curve on T-Mobile. Data is collected from San Francisco, New York, Dallas and Chicago and then aggregated to provide an overall monthly average in terms of both speed and reliability.