[unhingingdog]

I made a similar app (iOS) which varies the display time by word length, punctuation, and each word's place in a list of the 100 most common words (under the assumption that common words contain less information, thus take less effort to read). To be honest, I'm not sure it works any better than one running at a constant speed. There seems to be a surprising lack of research in this area.

(https://itunes.apple.com/us/app/zipf/id1366685837?mt=8 if you're interested.)