Microsoft recognized that Internet Explorer 9 needed to be fast. Web pages have more content every year and take longer to load. Increasingly, the browser replaces the function of an operating system -- it's a way to run Web-based applications and services. A slow browser is the kiss of death.
As a practical matter, there's no discernible difference between the top four finishers. A few ms here or there won't be noticed. So even though IE9 is faster on this test, it doesn't mean that in actual experience you'll notice it being faster. They all likely will perform similarly on those tasks measured by SunSpider.
So is IE9 the new speed king? Until objective tests about GPU acceleration are available, it's hard to know. But at a minimum, it's caught up to the competition, which is a big step forward for the browser.