@3Aliens
So OK you have a general question whether a big city on screen means everything slows down on any computer.
Yes it does.
But there are work-arounds.
You have probably seen matte paintings in movies to solve this kind of problem. Green Screen techniques also help.
You cheat. Just like Hollywood.
You might turn your attention to how the human eye works.
There is a camera technique referred to as 'bokeh' where the backdrop or background is deliberately blurred to focus attention on one element of a scene.
So you might see a forest or a city in the background and a small red balloon in the foreground.
The artist wishes you to pay attention to the balloon so they blur everything else.
And we humans accept this because that is how are eyes work anyway.
I am guessing that this is your question - how to deal with complex backdrops. And there are many cinematic work-arounds because we cannot deal with that level of complexity with our current technology.
So cheat.
Regards SeanMac