Here is simple scene, at early stage of project. But it's already noisy, and kind of slow thus. I attach different versions I tried to pin down the issue. Without any result.
I would gladly give the scene to somebody (Maru,etc..) who could have a look.
As you see, it took 1000 passes ( 2 hours on 160 threads for 2560px, small, preview render ) to get <1perc. noise ratio.
I then started stripping the scene by isolating only the building we see (image 1) for some increase, then diffuse only except windows (always thin mat, but still multiple panes), and last diffuse only, and no windows.
I don't think it gets dramatically less noisy.
I also tried portals, maybe I am stupid, but no matter how I use them, I cannot see any difference. Can I put them inside here ?