I think some commenters are attributing to Google an ulterior motive, whether ill- or good-intentioned, separate from its core business. But in this case no such motivation is necessary.
Basically, Google wants its users to be satisfied - otherwise it will lose to, say, Bing. So it measures user satisfaction - e.g., if a user clicks on a Google result, and immediately hits back button in three seconds, it's a strong signal that the user was not satisfied. And Google tries very hard to increase this "user satisfaction" (and other similar metrics), because not only does it help Google's business, but it also improves the service itself.
And, guess what? When a page takes fifteen seconds to load, lots of people hits the back (or close) button. Showing such a page is giving the user a bad experience. Unless there's literally no alternatives, it makes sense for Google to "penalize" such a page.
Of course no metric is perfect, so it will occasionally backfire and penalize a great page that takes thirty seconds to load. But that's life.