Making viewport frame windows without lag

Hi, I am currently making a roblox gta rp game and am making rooms for my building that will have rooms that are created under ground so that they won’t interfere with the actual buildings, I am trying to achieve a window affect that displays the whole city and people moving inside the window. I have noticed that if I were to clone the entire city and put it into a viewport, it would cause lag, and I am trying to be very lag efficient. Any help is welcome. Thank you

You could try and only reflect what is actually in view, so you can delete every object that is behind the mirror, and every object that is not visible due to occlusion (uses rays)

This is a complex issue

That wouldnt help with how big my map is going to be, im just trying to think if this is even possible


this is already how large it is