Does it happen for all types of tasks? Any reason you can think of?

I think all types of tasks.

I'm pretty sure I understand the reasons. Mostly cognitive biases and lack of focus.

My rational part of mind says that probably I'm an average. Maybe a little bit below due to low focus, but 100% I'm not the less productive.

But when I see how others work, emotional part of mind says that I'm the slowest person in the world.

That’s an extremely good point about rational/emotional — collecting real data points and looking at them from time to time may help overcome these biases (at least, it helped me and several colleagues in similar situations). Good luck! :-)