For a long time, two seconds was considered the maximum that Internet users should be expected to wait. Google and other companies find that now a barely perceptible delay isn't tolerated.

Wait a second.

No, that's too long.

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.

"Subconsciously, you don't like to wait," said Arvind Jain, a Google engineer who is the company's resident speed maestro. "Every millisecond matters."

Google and other technology companies are on a new quest for speed, challenging people like Mr. Jain to make fast go faster. The reason is that data-hungry smartphones and tablets are creating frustrating digital traffic jams, as people download maps, video clips of sports highlights, news updates or recommendations for nearby restaurants. The competition to be the quickest is fierce.

People will visit a Web site less often if it is slower than a close competitor by more than 250 milliseconds (a millisecond is a thousandth of a second).

"Two hundred fifty milliseconds, either slower or faster, is close to the magic number now for competitive advantage on the Web," said Harry Shum, a computer scientist and speed specialist at Microsoft.

The performances of Web sites vary, and so do users' expectations. A person will be more patient waiting for a video clip to load than for a search result. And Web sites constantly face trade-offs between visual richness and snappy response times. As entertainment and news sites, like The New York Times Web site, offer more video clips and interactive graphics, that depth can slow things down.

But speed matters in every context, research shows. Four out of five online users will click away if a video stalls while loading.

On a mobile phone, a Web page takes a leisurely nine seconds to load, according to Google, which tracks a huge range of sites. Download times on personal computers average about six seconds worldwide, and about 3.5 seconds on average in the United States. The major search engines, Google and, from Microsoft, Bing, are the speed demons of the Web, analysts say, typically delivering results in less than a second.

The hunger for speed on smartphones is a new business opportunity for companies like Akamai Technologies, which specializes in helping Web sites deliver services more quickly. Later this month, Akamai plans to introduce mobile accelerator software to help speed up the loading of a Web site or app.

The U.S. government recognizes the importance of speed in mobile computing. In February, Congress opened the door to an increase in network capacity for mobile devices, proposing legislation that permits the auctioning of public airwaves now used for television broadcasts to wireless Internet suppliers.

Overcoming speed bumps is part of the history of the Internet. …