Why are portal area lamps slowing down my renders? 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: 0:47
portals enabled: 1:36




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!


stage-render-portals.zip


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?)
Portals have limitations — it depends on how "open" your scene is

As mentioned by @Leander, the blender manual says that portals are not good for "outdoor" scenes.


portals lamps provide diminishing returns as your indoor scene becomes too "open"
in other words, if your indoor scene has several very large windows, you may be better off without using the portals (because the cost of portals starts to outweigh the benefits)
in my experience, I found portals to be unfit (by a narrow margin) for an indoor scene where I have a room with a whole wall basically missing (huge floor-to-ceiling window), and also an open doorway in one of the intact walls (think of a garage with the big door open, and also a side-door open too)