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

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?)

• I've found that when portals are enabled, the render is slower — however there is considerably less sampling noise.. to achieve the same level of noise with portals disabled, one must increase the sampling so much that the render time overtakes the portals-enabled scene — that is, portals slow down your render, but manages to "wring" out more bang for your buck for the same number of samples, and is thus worthwhile — TLDR portals let you lower the number of samples and achieve a faster render that way – ChaseMoskal Dec 7 '17 at 23:44
• While my above comment appears to be true for the portal-demo.zip scene I provided, it appears to me that there is a limitation on the usefulness of portals — they seem to have diminishing returns when the "openness" of your interior scene reaches a certain threshold — that is, in my scene with very large windows, portals appear to actually slow down the render and degrade the quality of the image (adding more apparent noise) – ChaseMoskal Dec 8 '17 at 0:15
• Yes, but all of the above has been stated before and is partially included in the manual. Use portals sparingly and only when the window is rather small. – Leander Dec 8 '17 at 0:16
• @Leander — thanks for including a link to the relevant area in the manual. It says portals aren't good for "outdoor" scenes, but that's fairly vague and I consider my scene rather indoors — for those who may be curious, my scene where portals are detracting from quality is a roughly cube-shaped roomm where one whole wall is simply open to the outdoors, and there is also an open doorway in an adjacent wall — this amount of "openness" seems to be about the crux point where portals are no longer worthwhile – ChaseMoskal Dec 8 '17 at 0:23