Google officially announced that they are including the site speed in the search ranking algorithms.
While site speed is a new signal, it doesn't carry as much weight as the relevance of a page. Currently, fewer than 1% of search queries are affected by the site speed signal in our implementation and the signal for site speed only applies for visitors searching in English on Google.com at this point. We launched this change a few weeks back after rigorous testing. If you haven't seen much change to your site rankings, then this site speed change possibly did not impact your site.
The post on the official Google blog also shows things that can help you to improve your site speed.
Read more here