Interior GI issue

[fa2020]

Hi,
In the demo scene of KeyShot called "area light" which contains a room and two cubes, when I render the scene using interior option on, all areas get cleaned but the ceilling take too long to clean and some stains(cloud like) remain there. The reason is that the GI bounce is on and has been set to something like 6. I know that for realistic interior scenes we need to increase GI bounces to a reasonable amount.
Is it a bug?
How to solve it?

[Rex]

The noise will smooth out with more samples.