I don't think there were ever any "slow updates" to Javascript, except maybe in the period of IE5/6 dominance to Firefox 1.0. In the Browser Wars, Microsoft and Netscape competed to add features as fast as possible. Firefox still has many additional features in its JS engine that aren't in ECMAScript (for each, coroutines).
I'm more concerned about adding features without fully considering the consequences. When you add a feature to Javascript, it's there forever. Rushed feature dumps are how we end up with things like the Web Audio API.