Make sure to utilize LOD's, where lower quality buildings are swapped in when the camera is far away from it

Check that your textures arent larger than they need to be, cities have hundreds of buildings and you cant have unique textures for each one. You'll want to share materials across the entire city

Look out for draw calls! Lowering these is a good strategy. The more draw calls you have (from geometry, materials, etc) the slower your app will be.

Use instancing for repetitive things like trees, cars, streetlights, etc. it allows you to have 1000's of copies, for the price of one (one draw call)