SAN FRANCISCO — In the rankings of the world’s most powerful supercomputers, a Japanese machine has earned the top spot with a performance that essentially laps the competition.

The computer, known as “K Computer,” is three times faster than a Chinese rival that previously held the top position, said Jack Dongarra, a professor of electrical engineering and computer science at the University of Tennessee at Knoxville who keeps the official rankings of computer performance.

K, built by Fujitsu and located at the Riken Advanced Institute for Computational Science in Kobe, Japan, represents a giant leap forward in speed. It will also undoubtedly be a source of national pride for Japan, at least among computer scientists, who take the race for fastest computer quite seriously.

“It’s a very impressive machine,” Mr. Dongarra said. “It’s a lot more powerful than the other computers.”