[consptheorist]

Are you suggesting that the remarkable disparity in performance was DOM specific?

Strange because I used a very common method appendChild() and I was under the impression that both browsers had optimized their respective inner workings a long time ago to the point that we should not notice such divergence in performance.

[dpe82]

Yes. DOM manipulation in all major browsers is implemented in C/C++. The JS engine is just a wrapper; any noticeable performance difference in DOM manipulation is almost certainly due to differences in the underlying layout engine and not in the JS engine.

[gsnedders]

Well, there's one way in which the JS engine effects it: how efficiently one can call into C++ from JS. Mozilla have done a lot of work to reduce the cost of that in SpiderMonkey.

[dpe82]

Sure, though in the grand scheme of things that penalty is pretty small when compared to the DOM operation itself. Eg. doing an appendChild() on an attached element and causing a reflow.

[gsnedders]

It depends a lot on what you're doing — if you're hitting fast-paths (esp. if you're dealing with out-of-tree nodes) it's entirely possible to end up with the JS/C++ trampoline being a significant part of the bottleneck, for much the same reasons as why Array.prototype.reduce can in several implementations.