It also comes from the fact that many jobs, and many more historically, are/were, in fact, paying you for your time. If you're fortunate enough to work in a job that doesn't care how much of your time is "company time," and you can work 5 hours a week to get everything expected of you done, that's great, but I would be quiet about it.
Any manager I've met would likely make a decision to give you 8x the amount expected of you each week, if that's your situation. That would indicate to me that we can find find someone less skilled that will take longer to complete objectives but we can pay significantly less, or we're not getting as much out of you as we're paying for.
Most people don't have the luxury you're describing, so I would hold on to that job situation!