That 40% "slacking off" is people working to capacity, and then refusing to work beyond it. Rather like mules, who will just stop working rather than let themselves be worked to death. Human brains do not work at full speed for 8+ hours at a time; 4-6 hours of intense concentration is about it, and that's for hyperfocused ADHD nerds like myself. Brains need downtime just like bodies; serious mental work is tiring.