You can't really load giga bytes of data in a web page without that page dying. The issue isn't really what Javascript can or cannot do (though speed is obviously a concern). Even if you manage to stream the data somehow you still need to persist it locally, and I'm pretty sure no browser allows that kind of quota. Photoshop feature complete is not going to run in the browser soon.

If what you said was true we would already have late Xbox 360 and PS3 like games running in the browser...with acceptable performances on high end hardware, which is clearly not the case.