I am not sure where to exactly put this, but I was one of the persons having issues when building the roof in on large structures where it would lag me out of the game! I think I found a sorta temporary solution that has seemed to help me a lot!
So you have a little house button named Change Vision Mode
Vision #1: Where you can see complete Structure
Vision #2: Where depending on where the camera is at the walls disappear
Vision #3: Where all the walls go down and all you see is Doors/Windows/Lamps and everything inside!
If you put it in Vision 3 while building roofs it reduces the lag by a whole bunch!! It has helped me out! So give it a try and tell me if it works!