I had lowered productivity a couple of periods from undetected health problems (allergy troubles and a bacterial infection).

I have been both a high producer and a problem producer. And I have surprised bosses in both good/bad ways, and they have changed their opinions about me.

The main personal difference was self confidence ("I burn out easy these days? :-( More exercise/relaxation don't help?! I am old?! :-(". Afterwards, my reaction was/is "So I was physically sick, I am not a complete luser that turn everything I touch to shit.")

The only difference in "dedication" was a later natural effect -- of burnout from working like an animal to get productivity when it was failing.

And the lesson I have learned is compassion; the cynical failures that are a depressing pain on us others might not be stupid or sociopaths. I am happy for the insight, it made me less of an asshole, but it was probably not worth the high cost.

Edit: To make this into a point, I do believe in the "Pygmalion effect" and I have seen it in action. But it is far from the whole story. There are probably also many other effects than bosses' behaviour and those I know personally which influence workers.