It doesn't perform better. But at the low end, you don't need to bootstrap a java VM (like Android) just to check your email and browse the web etc. You can do the little things on less hardware.
No, but you have to bring along a whole browser just to interpret code that checks the mail...
Anyway you spin it the low end that can't manage android will never in a million years be able to run an "OS" in javascript. There is a reason they demo it on a SGSII ;)
