Multicore is already mainstream and four cores are selling fast. IE9 platform preview 6 is here and it's still beating the other browsers on speed tests.
That's based on the average CPU reported by the Windows Experience Index score on all the Vista and Windows 7 PCs that run the performance test. And I noticed something watching Jason Weber show off how the latest IE9 platform preview is still faster than the nightly builds of Firefox and Chrome - which answers some of the criticisms about the IE team cherrypicking which browsers to compare.
The latest Chrome 8 nightly with the default hardware acceleration enabled runs the Speed Read test at a third of the IE9 speed (20fps compared to 60fps); IE9 finishes the test in 14 seconds, Chrome took around six minutes to run on the same PC and at 3fps Safari actually takes around 18 hours to finish the test.
On the Pacman-style Browser Hunt test that IE9 was running at 60fps thanks to its GPU-accelerated canvas, the Chrome 9 build from 7.30 this morning only managed 38 to 45fps for example, and the latest Firefox 4 beta 6 nightly build flew the SVG helicopter at 18mh rather than 180mph, with the movement looking very jerky - although the average is 17fps, it was taking anywhere between 34 and 88 milliseconds to draw a frame. Firefox shows the same variance in times in the fish tank test; the variation in how long Firefox takes t do a call back is two or three times more than the variance from IE, so again your experience isn't as smooth - which could get frustrating in a complex Web app.
On February 1st this year, the average number of cores was 2.3 (up from 2 in December 2009). By September that was up to 2.42 cores and the average core number on October 1st was 2.46 cores. With all the single-core Atom netbooks pulling down the average, there's still a lot of four cores and more on the way.