Google Inc.'s new beta of its Chrome browser for the Mac is nearly twice as fast as Mozilla Corp.'s Firefox, but it can't keep up with Apple Inc.'s Safari, benchmark tests show.

According to tests run by Computerworld, the Chrome beta, which Google launched yesterday, is the second fastest of four Mac browsers tested. Chrome renders JavaScript 10 times faster than Opera 10.10 and almost twice as fast as Firefox 3.6 Beta 4, the most recently released Mac version of Mozilla's open-source browser.

But Chrome can't match Safari 4.0.4's speed: Apple's browser is approximately 12% faster than Google's beta.

That's different than the situation on Windows, where Chrome has been billed as the fastest browser around. The last time Computerworld tested Chrome on Windows -- in September, with Chrome 3.0 -- Google's browser handily beat all rivals, including second-place Safari.

Computerworld ran the SunSpider JavaScript benchmark suite in Mac OS X 10.6 three times for each browser, then averaged the scores.

Google made no direct JavaScript speed claims for its Mac beta Tuesday, as it has regularly with its Windows browser. Instead, the company bragged about Chrome's quick start-up on the Mac. "As you might expect, the speed of Google Chrome for Mac is something we're very proud of," said Brian Rakowski, a Chrome product manager, in a post to Google's blog. "Try installing the beta and see how fast it launches -- there's hardly even time for the icon in the dock to bounce!"

Most browser makers have been aggressively promoting improved JavaScript performance for over a year and a half, when Mozilla began touting the performance boost that its new TraceMonkey JavaScript engine provided to Firefox 3.5. Only Microsoft Corp. has disparaged such benchmarks, calling them nothing more than a "browser drag race" that doesn't accurately portray real-world use.

But even Microsoft has seen the JavaScript light. Last month, Microsoft -- which hasn't made a Mac browser since 2003 -- promised that Internet Explorer 9 would close the JavaScript gap and claimed that SunSpider scores for its work-in-progress browser were "very close" to those of its Windows rivals.

Chrome currently accounts for approximately 4% of all browsers used worldwide, according to the most recent data from Web metrics company Net Applications. The vast majority of Chrome's share, however, is on Windows; before Tuesday, only rougher development builds had been available for the Mac.

In comparison, IE has a 64% share and Mozilla's Firefox has a 25%. Only Opera, with just 2% of the browser usage share, trails Chrome.

The Chrome Mac beta can be downloaded from Google's Web site. It requires Mac OS X 10.5, a.k.a. Leopard, or later, and runs only on Intel-based Macs.