Go look at any hands on video or flash it yourself on a device that runs Android. It's clear as day that Firefox OS is far from fast and responsive on hardware that Android doesn't have issues with. And this makes a ton of logical sense, as it would be a goddamn miracle if Mozilla managed to make web technologies an order of magnitude faster and more memory efficient than anyone else.

Or hell, just run Firefox on your existing Android device - that's all Firefox OS is in reality. Mozilla makes a bunch of claims about how it's "closer to the metal" or some such nonsense, but it isn't. It's not like Android runs all native code in a VM - it's as "close to the metal" as you can get, with the exception that system calls go to the kernel. Just as they do in Firefox OS. Particularly since Firefox OS is built on top of Android.