Ironically, it seems to me that the happiest grad students are the ones who are hard working and graduate quickly (~5 years). It's the "relaxed" students who spend 7+ years in the PhD who seem the most jaded and unhappy. When you say you want to be in a relaxed program, what exactly appeals to you?
As an example, I went to the visit weekend for UIUC's CS PhD, a top-ranked and well known program. Their students work hard, and almost all of them graduate within 5 years. They also happened to be the happiest PhD students I have ever met. My hypothesis is that by maintaining a fast pace, they stay more motivated and enjoy the program, rather than letting it drag on without a clear graduation timeline.