Is it immoral to only spend 50% of the time at work productively?

I feel that people often make a big deal of somebody using 5 minutes of Facebook a day during the boss' time. I have the feeling that the average person only works 3 hours effective per hour, most people don't do that much and lie to themselves.

Should I feel bad for feeling that I should not feel bad about spending 4 hours a day productively as I still seem to get paid more and be more productive than most other colleagues.

At school I was told that when people start working their office work, that they work so hard and people have it so bad, but I feel lots of us are being extremely moralistic and hyper-critical of others, making them feel quilty, while being quiet lazy themselves.

Should I really work my ass of 8 hours per hour or should I just feel good with myself if with working 4 hours per week the results are better than average anyway?