With portals, you can get away with lower lightmass settings, and not messing with the ini as much. I would still use them for every window, and try lowering Indirect Lighting quality to speed up render times. It should be in the middle of the window, and take up the entire length of the window, and be used for every window where you want higher quality lighting and shadows inside. It should be slightly larger than the window, and can be used on big windows.