Thank you for the reply and the rendering test.
It wasn't the skylight/hdri the enemy...
Finally I have found what was the problem.
I will share my discover here because these forums and in general
all 3d forums are the biggest help and the greatest school for us
that we are spending tons of hour behind a computer...
So after that you rendered the image in such a quick time I discovered
that the problem was with the lights!!!!!!!!
The scene I sent you -except the Mrsun- has no lights inside the house.
If you create for ex. 15 photometric lights inside this house and render again,
you will see that the render is very slow.
So what I did?
I excluded all the Forest Pack objects from the lights and voila'!
The render has become a lot faster....