Google Engineer Matt Cutts has recently told Web Pro News in an interview about how Google may rank websites in future. He has hinted that ranking would somehow be affected by the loading time taken by a website.
Here's an extract from the interview:
Matt Cutts in his statement has made it clear that the loading time taken by website plays a major role in getting visitors. If the website loads slowly, potential visitors will be driven away whereas, a faster site will make them stay longer. Google has been putting in efforts to make the Internet surfing experience a bit faster for its users.
It is not sure if Google will consider site speed for ranking a website, but if it does, site owners will definitely require a site speed optimization, SSO along with SEO.