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 ;)

They had another device running it which really was much worse then the SGSII, but with a much smaller screen it doesn't make for great video.

Dalvik being a bigger overhead than JägerMonkey?

Your comment should be sitting higher. Dalvik is unlikely to be more expensive than a similarly complex JS VM.