Google warned us this would happen and beginning today, landing page load times will affect Adwords Quality Score. The reason behind the change is something that Google often touts: it's what's best for the user. Google argues that faster load times are better for users, so pages that load faster will be rewarded accordingly.
In May, the Adwords team launched a load time metric to help you evaluate your pages.
And you might be surprised what you can do to improve your load times - and increase your conversions. Recently, the Website Optimizer team suggested testing best practices. They gave an example of a page that removed an image of a customer service rep. Best practices said leave the picture up, but the tests showed conversions increased by leaving the picture out. Of course, images generally increase load times.
Before you do any drastic changes to your pages so they'll load faster, testing your new page can help you know whether or not your changes will convert.