Google uses a multitude of factors to determine how to rank search engine results. Typically, these factors are either related to the content of a webpage (the text, its URL, the titles and headers, etc.) or are measurements of the authenticity of the website itself (age of the domain name, number and quality of inbound links, etc.).
However, in 2010, Google did something very different. Google announced website speed would begin having an impact on search ranking. Now, the speed at which a website/webpage loads the content from a search result is a ranking factor.
If you're a WordPress user, I can make your life easier. You can find almost all themes from the WordPress.org theme repository tested against Google PageSpeed here. Enjoy!