Well there are several 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 