Economic Times: Web users aren't willing to wait an extra second or two for Web pages to load anymore. In fact, Google researchers have found that a delay as short as 400 ms, literally the time it takes to blink an eye, can cause Internet users to change their habits. According to Microsoft's Harry Shum, "Two hundred fifty milliseconds, either slower or faster, is close to the magic number now for competitive advantage on the Web." In other words, customers will switch to a competitor's service or site if it is just 250 ms faster than the one they are currently using.
Because they know how much speed matters, most search engines, including Microsoft's Bing and Google, load in less than a second. The average for other sites is 3.5 seconds for U.S. users on PCs and 9 seconds for those on mobile phones.