Can someone explain what happening in the backstage when switching layouts?

First start by fixing the root problem which, in my opinion, is the fact that there are entities that have different Z values scattered throughout the drawing. Who (what company) is ultimately responsible for the drawing? Is it the company that you work for?

First start by fixing the root problem which, in my opinion, is the fact that there are entities that have different Z values scattered throughout the drawing. Who (what company) is ultimately responsible for the drawing? Is it the company that you work for?

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From "other office"? Do you mean another office of the same company you work for? I'm not suggesting you exercise "control" over their cad management rather I am suggesting you have a conversation with whoever is in charge of their cad techs and explain the nature of the problems that consistently crop up with their drawings. Work that is done incorrectly negatively affects the bottom line. You are in business to make money right? Why should you be wasting your time, effort and money having to correct the same types of mistakes again and again? If this "other office" is a part of the company you work for then upper management needs to know there is a problem. If it is a totally different/separate company from yours perhaps you'd be better off finding someone else to partner with. There is no room for shoddy workmanship no matter if you are an architect, a mason or a cad tech.

Like Remark plot out the model spaces as 1 sheet each every dwg, xrefs included layou on the floor and get a fat pen and look at the way all the parts are intertwined and duplicated and plain junk, remove some of these links organise your xref tree better and your speed will come back.

Look at a tree your dwg is at the base the the xrefs go up the branches, they do not inter merge with each other.

We have repeatedly said its not your computer, its your CAD management. Testing at home I can not help as it grinds my laptop to a halt.