It does happen that some of the most skilled virtual machine developers in the world are working on V8, for example. (Lars Bak is leading their effort, and he was one of the leads on the Self/Java Hotspot engines, having now built several generations of optimizers for dynamic languages). This is not a coincidence. ;-)
And it also happens that, until a little while back when JS performance became important, the JS engines were pretty backwater-ish, and quite slow. So it's fairly easy to make important speed gains when starting from such a lowly position.