I already replied to a topic like this, by the way those are some ways to reduce lag:
- Remove the terrain that you don’t see
- Enable streaming (for more informations Content streaming )
- Disable Cast Shadows in the parts that doesn’t need shadows
- Optimize your scripts
- Use meshes (how to create a mesh from a building you did in studio meshes )
- Optimise the collions box (for other building tips here )
- Remove Can Collide to parts that you can’t touch
Hope it helps 
PS: Sorry for the bad English 