Confined spaces

I dont know why this happens but when I make a confined space
such as a building with a character inside the game slows down but,
when I play other peoples games which have the same it doesn’t slow down! :x

Anybody know why?

it isn’t much the confined space but the objects in or outside the space. If all you ahve is a big cube and a character and it runs slow, i don’t know.

Pooba