I hope to find out the real difference in time of this optimization method. In my interior, everything is built on boxes, there are hundreds of them, they almost always have only one side visible. I'll try to remove everything unnecessary.

Removing all invisible sides on hundreds of boxes will certainly decrease the overall size of your project.
The difference in rendering time depends on how smart the renderer is. A very good renderer will not render faces that are not visible but I think most just render everything that is in the objects, thus rendering visible faces over invisible faces. In that case the rendering time should also decrease significantly.

By-the-way: great usage of the photo-on-box for the chandelier!