I never really thought of "sluggish" as a synonym or part of the definition for phlegmatic. Maybe because, for whatever reason, phlegmatic has a positive association for me. I tend to think of Ron Swanson when I hear "phlegmatic."
Found this word when researching Judge Selya of the First Circuit. Despite almost every other judge in the history of time (with the exception of Judge Kozinski), Selya's decisions are beautifully written. You can see an example of his style in this recent case, concerning Facebook.