Actually JS performance isn't the bottleneck in the vast majority of cases now. JS is more than fast enough, it's the DOM that's slow. And that is what Servo is going to help with.

And as for Firefox on Android, I have plenty of hope for it. I'm seeing more and more people switch to alternative browsers for speed (the most common one is samsung's "browser" which everyone says is "super fast" but really only is a weird hack to make scrolling smooth which breaks a few standards).

Ios is another story, but at least on android if they make a damn good product, people will use it.