My scene is lit entirely with emission surfaces, and I saw no difference by adding a light portal. Changing samples didn’t make any difference, and rendering with the light not in the camera didn’t make a difference either.
Before:
After:
I put a light portal of greater width than the sphere on the pedestal, and tracked it to the camera to ensure it was facing the correct direction. Does this only help when the light is from outside the area coming through a small opening?
Edit: I asked if the light source being outside made a difference, then realized I could just test that. The short answer is it did make a difference, however the non-portal version seems to be a little cleaner at 20 samples.
No Portal @ 20 samples:
With Portal at 20 samples:
At a lower sample rate (10 samples) the difference was even more pronounced, which also surprised me.
No Portal:
With Portal:
Edit 2: With World Lighting. Here the light portal made a huuuge difference. Removed all emmissive surfaces from render view and just used world lighting.
Before:
After:
