The more the lights are, the more the noize??

Hi guys,

I have a pretty weird lighting problem.
I have a city scene with an HDR and 4 area lights inside a building. Everything looks nice with dense lighting and smooth shadows, as you can see in this picture:


But as I add several other area lights outside, the lighting in the building gets awfully noisy (!!!).


How could this be? Both renders have 600 samples (Blender Cycles, 2.79).

Here are my render settings:


Could be important to specify that the area light inside the building are part of the project, the other area lights that I added in the second render are instanced from another project.

Many thanks in advance to who might help me :slight_smile: