I really do not understand why this push for js (such as for themes). I loaded this theme up in my local test site and the front-end blanked out; try doing a theme check (longggg list of errors)...

As for speed, that is subjective considering many sites will have a ton of plugins, media like videos, images, graphics, and other forms of content. Don't forget, a lot of end-users customize their themes; expecting a person who has limited coding skills to handle a JS theme? I think not. Sorry, but I think this whole thing about JS is a fad unless you are creating website applications and/or plugins. PHP themes will not be disappearing anytime soon.