In site usability, ecommerce, and conversion in general speed is well established as being very important. Google and others have made speed a core goal because

In site usability, ecommerce, and conversion in general speed is well established as being very important. Google and others have made speed a core goal because their data shows that speed helps conversion. However, I would suggest that site speed is merely a proxy for usability speed. ie. it's more important for conversion to have a form that is easily comprehended and filled out than it is to have a fast loading page. What I'm working up to is the idea that usability speed might be a useful proxy for a high-conversion design. It's also data that can be measured accurately without bias, and can be generated in lieu of real visitors by throwing a horde of disinterested people from Mechanical Turk at (eg.) your sign-up page with a well-defined goal.