I'm intrigued that they keep pointing out that it's not using any asm.js optimisations, but still "performs well".

"Performing well" in this case still means significantly slower than Firefox does on the same code:

http://arewefastyet.com/#machine=12&view=breakdown&suite=asm...

Which isn't to say that it's not a big improvement over where it was. But there's still a way to go on making it competitive, so far as I can see.

Edit: Got name of the original engine wrong.