I was confused when I added portals to my scene, and it actually increased (slowed down) the render quite considerably!
Here is the demo scene I used to confirm this: portal-demo.zip
- portals disabled:
- portals enabled:
Additional edits after original post
In the scene I'm working on, I created two renders to compare portal lamps enabled/disabled — the sample amount is adjusted to achieve the same render time — and I find that the scene with portals enabled looks considerably worse!
This indicates to me, that in my scene, the portals cause the render to slow down, and ultimately produces a worse result.
Could this be, because my room has two rather large windows, and with a certain amount of "openness" to your indoor scene, perhaps the benefits of portals are outweighed by the costs?
(my previous little suzanne
portal-demo.zip room seems more reasonable, perhaps because it has a much smaller window?)