1, why would you change intensities of corona sun and sky?
2, There is some background going on behind window. If that's geo with some material, that could be a part of the problem.
3, You seem to have used some material converter. There might be a lot of materials done wrong and slow
4, Do you have glass in your windows?
5, Any chance you could share (privately) the scene, so i or someone else can help you set it up?