A third of all American adults own a smartphone and for many minority and low income users, those mobile devices have replaced computers for Internet access.

The findings released Monday by the Pew Internet & American Life Project highlight the breakneck speed consumers are adopting smartphones — faster than just about any high-tech product in history.

It’s been four years since the introduction of the iPhone and rival devices that run Google’s Android software. In that time, the devices have turned much of America into an always-on, Internet-on-the-go society.

A quarter of Americans with smartphones use the devices as their main way to get onto the Internet, the Pew study found. About nine in 10 owners of such devices access the Web and check their e-mail each day through their device.

Smartphone users are diverse. Most are well-off and educated. And, adoption by blacks and Hispanics is particularly high at 44 percent.

“For businesses, government agencies and nonprofits who want to engage with certain communities, they will find them in front of a four-inch screen, not in front of a big computer in their den,” said Aaron Smith, a researcher at Pew and author of the report.

The size of the screen is just about the only thing that keeps Miguel Reyes, 20, on his laptop. He prefers to watch soccer matches and play video games on his Mac. But for just about everything else, his iPhone will do.

The recent high-school graduate from Landover was lounging in his backyard last month and was reminded about a book he wanted from Mexican author Francisco Jimenez. Instead of walking into his bedroom and opening his laptop, he grabbed his iPhone from his pocket, fired up his browser and looked up prices for the book on different sites.

“I’m finding fewer and fewer things that make my laptop all that much better than my phone,” Reyes said.

Of those who solely rely on smartphones to surf the Web, most are minorities, younger than 30 and have low incomes. They’ve found mobile devices as a suitable replacement for buying expensive computers and paying DSL or cable modem bills every month, Smith said. Cable and DSL remain faster, but that difference may not be big enough to justify their high costs for some consumers.

Currently, 63 percent of all Web traffic is generated by computers, but that number will decline to 46 percent by 2015, according to Cisco, the Internet network equipment maker.

“Speeds are improving, WiFi is getting faster and devices are proliferating,” Cisco vice president of policy Mary L. Brown said.

By 2015, the number of mobile Internet users around the globe will increase 56-fold to 788 million users, Cisco said.

Boosting their numbers will be fourth-generation high-speed Internet networks, transmitted over new cell towers, with speeds that compare to some DSL connections, experts say. That means users will be able to watch high-definition streaming video on smartphones with few interruptions.

Right now, smartphone users can have an even better video experience on WiFi connections. Many smartphone users in urban areas already rely on a “hot spot” that offers WiFi Web access from a school, library or a neighbor to get onto the Web cheaply, industry analysts say.

Yet smartphones have limitations.

Filling out a medical form or a job application and creating presentations for class or work is difficult because of the way the content is formatted on devices.

And recent changes by wireless carriers on the way users are billed may discourage smartphone users from cutting the cord to their cable or DSL provider.

Verizon, for instance, is charging new customers for every bit of data they consume, rather than offering all-you-can-eat monthly plans.

Some public interest groups say those plans are designed to keep users in bundled packages for multiple services. AT&T and Verizon, for instance, offer wireless, broadband home Internet and television services.

“In many cases these wireless carriers also have wireline and television services and what they want is to encourage users to keep using all three services, not to replace one for the other,” said Benjamin Lennett, a senior technology policy analyst at the New America Foundation.