Fake Productivity

Oooooh I knew this feeling so well at my job. My solution was to quit and move to open-end-date hourly work (aka permalance) at a higher hourly rate than I was really comfortable with. When I wasn't actually working for the client, my ethics demanded I turn off the timer. When I had the timer on, the hourly rate demanded I bust my butt working really hard. This was transformative for me on two fronts. First, it meant that every hour I was "working" had this strong pressure to keep working really hard and stay productive. Second, it reduced the number of hours I worked each week to about 25, which was fantastic for my stress levels.

I know that's not a perfect or even possible for everyone, but I think there are two key takeaways from that experience that you can apply even if you're still working a 9-5 office job.

First takeaway: it's ok to not be productive all the time. Accepting there were hours I couldn't be productive and just needed a break really made me a lot happier. The tricky thing here, as you've said in a few other replies, is how to use that time in the bounds of traditional employment. There's options like personal development or picking up meditation. If you work in a city, you can start taking 30 minute walk breaks every other hour. Indeed, if you really find you are just working 5 hours a day but getting everything done, you can ask your manager about changing the expectations of your work environment - maybe you can take extra long lunches or leave an hour earlier each evening. The worst answer you can get for asking for something like that is a "no" and, if so, that's just one more datapoint to consider when you next think about leaving this job.

Second takeaway: track your time. Even if you're salaried, you can sign up for a tool like Toggl Free and track how much of your day is spent on tasks like coding, personal development, and meetings vs just being in the office filling space. Even if just for yourself, having this tracked and having the external reinforcement of "if I'm going to start goofing off, I can do that but I have to switch Toggl first" helps build boundaries between those activities and make it easier to click into and stay in each mode. This was my favorite change about becoming self-employed and one I'll definitely keep even if I work at a traditional company again.

This is exactly what I experienced on my previous job. I tried to finish with all my tasks as soon as possible and after that spent time to focus on learning and building relationship.

I also think that this is ok to be productive in coding/programming only 3-5 hours per day, next morning is always better time to deal with difficult stuff

Depends on what you mean by productive. If you only need 3 out of 8 hours to complete your daily tasks it means you can:
a) Polish the sh*t out of the tasks you do. Make them perfect, better than needed.
b) Fill the other 5 hours with learning new skills.
c) Tell your boss that the work that was planned for you to do is not challenging enough and ask for something harder. This way you might still only be able to do 3 hours of productive work, but the other 5 will be used to find a good solution for your current tasks.

The bigger question is who is this a problem for? For you? For your boss? For your company?