Not sure if this is what KevinGrant was referring to, but this article discusses the same phenomenon

http://rosettaproject.org/blog/02012/mar/1/language-speed-vs-density/

Sorry, I don't have a link for it. The result is just something that I remember reading about many years ago. I looked at the link that redding posted and while it probably isn't the same paper (I think I read about this before 2011) the result seems to match what I remember. There's a possibility that if the linked paper could be retrieved, then whatever I read may be in the bibliography, although I don't know if I'd recognize it as such.