Looking at their v8 bench speedup: from 0.7 ms to around 0.3 ms we get again 2.3, comparable with the simple loop measurement. But interestingly enough, Google's V8 gets its speed in a very different way, as my mentioned benchmark loop performs much worse in Chrome: 3.9 sec on Chrome 6.0.472, thats 4.4 times slower than ActionScript or Firefox Beta 4.