Google made it clear some time ago that it wanted to take into account the ‘usability’ of a website when determining where it ranks its pages.
To do this, Google decided to take into account the speed at which web pages load. In other words, a slow site is less usable and a quick site is more usable. Obviously, this doesn’t take into account the extent to which a site’s layout and functionality aid usability, but for Google, the site’s speed is a good place to start.
It’s worth saying upfront that a website’s load speed will not be a critical factor that decides whether your site appears on page 1 or, say, page 10. There are other factors that are much more influential (including your domain’s age, your site’s content/coding, and the number and quality of other sites that link to your site). But, if you are already on top of those aspects of SEO, you should consider its speed as well.