Leading Web sites, such as Google, Facebook, and MapQuest, are struggling to perfect their mobile offerings, especially when it comes to performance, according to a report released on Thursday by Keynote Competitive Research.

It's estimated that there are nearly 3 billion cell phone users in the world, many of which are starting to embrace the mobile Web. So it's more important than ever for popular Web sites to optimize their content for use on cell phone browsers.

Keynote's new mobile Web performance index measured and ranked several popular Web sites based on response time, download speed, and success rate.

Google ranked the highest in terms of response time, but scored in the bottom half for download speed. Response time refers to the average time in seconds it took to download a mobile Web site to a cell phone.

Google's response time averaged 6.6 seconds, followed by Facebook with 6.7 seconds. Yahoo proved to have a much slower response time than Google, averaging 9 seconds. CNN, MapQuest, MSN, and ESPN all ranked very low with response time over 10 seconds.

Keynote measured other Web sites, including Fandango, Schwab, and the Weather Channel, which all provided a response time of less than 10 seconds.

Download speed, meaning the average speed (in bytes per second) at which a mobile Web site downloaded to a cell phone, was another benchmark used by Keynote to evaluate the Web sites. CNN ranked the highest with an average download speed of 2,368 bytes per second, followed by ESPN with 1,756 bps, and MSN with 1,484 bps.

MapQuest, Yahoo, Google, Fandango, the Weather Channel, Schwab, and Facebook all scored low, offering download speeds of less than 1,300 bps.

While most of these Web sites are able to successfully display full content on a cell phone, they can use some improvements. Keynote placed Facebook and Google as Web sites with the highest success rate, meaning users were able to download their content to a cell phone 99% of the time. MSN scored the lowest with 97%.