From other things deriving I would guess, that veteran users become more efficient, looking for what they really care about instead of letting their eyes and brain parse every letter. It's the same for me here on HN. I don't care about 90% of the content, even from the front page. So I don't even read it just scroll until my eyes find words they consider interesting. But I spend more time researching other sources to an interesting topic and actually spent some meaningful time writing meaningful comments (or at least try to).

edit I even end up more often these days using the browser search function to not need to read every entry.