well, first, you gotta know that games like gta 3(or vice city) are made as loose modals. Every part of the "city" is a modal. As in, every building and stuff. For this to work in 3d gamestudio you could make models (of buildings) or map entities. And in one map just glue 'm all togetter, without using blocks. Just the models. You will see that the build time will be around 1 or 2 secs. No matter how big your map is. And it will work. Even on low-end pc's. cuz modals are loaded on sight and aren't hanging around in the video memory when they are out of sight for the player. (blocks and prefabs do that)

Worked for me