Do the Polycount, check the number of polygons in the model.

Try setting all your windows and doors to scale sensitive, hide all the layers that you actually don't see, remove doorknobs etc, simple 3D for the trees ....

Then, set your 3D view to a 1:500 or 1:1000 scale.

Do the Polycount again. If the number is significantly down, you are making progress! The Polycount will list all the used library parts, maybe you can make them even less complex. Check the parameters.

And so on ...