A slow website loses customers. Period. That’s why Google employee Steve Souders, author of the definitive O’Reilly manuals High Performance Web Sites and Even Faster Web Sites, did a performance test of nine popular widgets for websites, to see how much they slowed down the host’s page loads.

He ran all nine — Digg, Facebook, Google Analytics, Google AdSense, BuySellAds, ValueClick, Quantcast, Collective Media and Glam Media — through Google’s Page Speed and Yahoo’s YSlow benchmark tools and posted the full table of results on his personal website.

Souder found that Digg’s widget was the biggest boat anchor of the bunch, adding two-thirds of a second to page load times. Facebook Sharer, the second worst, added about one-sixth of a second. These times seem trivial, but many sites embed multiple widgets in their pages. The total adds up to far more than the 400 millisecond delay that Google found would lose them several hundred million dollars a year in ad revenue.

Not all of the advertising widgets were testable, because, Souder wrote, “it’s not possible to gather timing stats for snippets with live ads.”

Beyond the sheer number-of-milliseconds delay, Souder created an Impact on Page rating that included “content size, number of HTTP requests, and performance issues,” all of which are easily observed by non-technical users.

Ad networks had the biggest impact in his tests, with all but BuySellAds rating a “medium” or “big” impact score. Souders wrote earlier this year that “the performance problem of ads” was a hot topic among site engineers trying to extract the best performance and, by extension, the most business from their sites.