Only Google's own words:

"We believe that making our websites load and display faster improves the user's experience and helps them become more productive"googlewebmastercentral.blogspot.com
"Users value ads that bring them to the information they want as efficiently as possible. A high-quality landing page should have a fast load time as well as feature unique, relevant content. Fast load times benefit advertisers as well, since users are less likely to abandon a site that loads quickly"http://adwords.google.com/support/
"We've always viewed speed as a competitive advantage. Because the cost of slower performance increases over time and persists, we encourage site designers to think twice about adding a feature that hurts performance if the benefit of the feature is unproven"http://googleresearch.blogspot.com
"We're including a new signal in our search ranking algorithms: site speed. Site speed reflects how quickly a website responds to web requests.

Speeding up websites is important - not just to site owners, but to all Internet users. Faster sites create happy users, and we've seen in our internal studies that when a site responds slowly, visitors spend less time there. But, faster sites don't just improve user experience; recent data shows that improving site speed also reduces operating costs"
http://googleresearch.blogspot.com


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