If you are using Max I posted this last weekend:
And just to clarify, is quite right, however you need to be careful with the level of granularity of your modular pieces. Even though they will save render time, the number of objects will increase which will impact the CPU. It’s a matter of finding a sweet spot
If your building it’s going to be seen from street level, you most likely want to keep the ground level and first/ second floor modular, them as soon as possible make the higher up floors into bigger meshes and bake the geometry details into a normal map and a height map.
Also if you are building a large city scene, LODs are your friends.
Occlusion is going to play one of the most important roles and yes, putting everything together in a DCC app and exporting it “in place” is BAD idea.