Something we all knew is now official policy:

You may have heard that here at Google we’re obsessed with speed, in our products and on the web. As part of that effort, today we’re including a new signal in our search ranking algorithms: site speed. Site speed reflects how quickly a website responds to web requests…

…If you are a site owner, webmaster or a web author, here are some free tools that you can use to evaluate the speed of your site:

• Page Speed, an open source Firefox/Firebug add-on that evaluates the performance of web pages and gives suggestions for improvement.
• YSlow, a free tool from Yahoo! that suggests ways to improve website speed.
• WebPagetest shows a waterfall view of your pages’ load performance plus an optimization checklist.
• In Webmaster Tools, Labs > Site Performance shows the speed of your website as experienced by users around the world as in the chart below. We’ve also blogged about site performance.