No one wants to create a slow site. Yet that's exactly what happens. Not only do fat sites exist, they're actually becoming more and more common. It would be one thing if performance was merely another feature, but the reality is that performance is a fundamental component of the user experience. As Steve Jobs once pointed out, design is not merely about how something looks but also how something works.
If we want to start providing people with the fast experiences they so desperately want, performance needs to be engrained throughout the entire process.