[shawndrost]

This is not just some art thing. People note it in every field. It's not the only predictor; the author of Chrome reportedly did most of it offline IIRC. But it's a real phenomenon that is robust in the face of the concerns you're raising. There is a wide range of email responsiveness -- even among people who are going to the bathroom and putting on birthday parties and doing focus work -- and it is a helpful predictor.