Steve Lohr for the New York Times:
Remember when you were willing to wait a few seconds for a computer to respond to a click on a Web site or a tap on a keyboard? These days, even 400 milliseconds — literally the blink of an eye — is too long, as Google engineers have discovered. That barely perceptible delay causes people to search less.
Google’s research indicates that people will visit a Web site less frequently if it’s slower than a competitor’s page to load by 250 milliseconds. This is important to keep in mind for anyone doing Web publishing these days, as this isn’t just a factor of how powerful your servers are, but how well laid out your pages are, how optimized the graphics are, and how quick the ads are to load.