Keeping one’s website up and loading quickly is vitally important for any company, but for the 500 biggest companies in the world, poor website load times can be an especially costly proposition. For the largest companies, negative brand associations due to poor experiences from slow loading websites can ultimately cost millions of dollars.

That’s why brands definitely don’t want to be on the list of worst offenders when it comes to website loading times. New Zealand-based web optimization company Aptimize just released its 2010 Website Performance Benchmarks whitepaper [PDF], which is based on the averages of the load times of all Fortune 500 companies’ websites.

Below is an infographic depicting the worst transgressors. The size of the logo correlates to the slowness of their respective sites — the bigger the logo, the worse the user experience. Surprisingly, some large consumer brands — including Nationwide auto insurance, CVS, Coors and DollarTree — are among the worst big company websites. Even Southwest, a company known for its social media savvy, didn’t fare so well in the Aptimize survey.

Authored by Josh Catone of www.Mashable.com