Despite my best efforts in looking all over for this information, I still can’t seem to find a ruling on the effectiveness of portals in a practical setting.
It seems that the effectiveness CAN bee seen under very controlled circumstances, as in a pin-hole window type scenario. But for the life of me, for any typical interior scene, I always see a degradation in quality (in my renders at least). I very well could be doing something wrong. I most likely am. I always understood portals as guiding light rays to/through window openings.
First picture is the scene, second is a closeup comparison, and then my file at the end. The HDRI used can be anything with a high-noon sun.
house2.blend (1.5 MB)