If there one guiding principle present on every Google-owned project, that is “speed”. As thinking of speed inherently provides the necessary perspective that balances form and function, so to speak. It’s all about user expericence, let me explain:
Think about a very nice webpage, that blows your mind visually, but that’s painfully slow to load. You use that page to get some information (be it view videos or new photos, be it a news service, a place where you buy something, etc). Now think about a bare bones, old looking website, that has similar information or products, but it is razor super fast. Now, where would you go next time?
Now think marketing: What website would you want to be recommended? Or, would you recomend the slow website?
And now think business: Where would you place your ads if you had the choice? What is the website that has the chances to get more page-views, and as such, the one that gets the most exposition and more revenue generation opportunities?
Now… see why speed is a Google search ranking factor?
The field of web performance is a complex space. There are many interesting books, blog posts and discussion forums. mod_pagespeed is a project at Google that assembles a good set of pages related to web performance and organizes it in terms of contribution to the speed of a web page, whether by serving less bytes -through better compression or caching-, minimizing round-trips and optimizing the order of resource download for the browser.
They are assembling a very nice Performance Guide, updated regularly with new rules and updates to how the rules apply to modern browsers.