Have any evidence that Firefox performance is "atrocious"? Firefox JavaScript is faster than any other browser except Chrome (and beats Chrome on some apps like JSLint), and it has GPU acceleration while Chrome does not.
No, I don't have benchmarks. I was astonished at the relative speed Chrome had a good while ago, and I switched over.
It might be as simple as good UX engineering to enhance perception of performance while other things happen in the background. I have no idea. But on {Win|Mac|Linux}, Chrome just zooms.
Because Firefox performance is atrocious, and that is the single most salient point from the Mozilla project.