Portal Lights + GI

[Druski]

It would appear that portal lights don't work unless the emmisive/self-illuminated object is generating GI. In a recent interior lighting scenario, where I have tried to leverage portal lights to derive light colour and intensity from HDRI projections, my render-times have been huge. If the portal lights worked without relying on these surfaces generating GI, my setups would have been infinitely faster. I can get around the issue by baking all the textures for the lights (which is quite tedious), but the portal light workflow is really fast and intuitive, so obviously my preference.

Is there any reason why this is the case? Can it be adapted to work without GI to increase efficiency?

[yolov]

I'm not sure I fully understand the setup. Is there something very peculiar you're trying to do with portal lights that's (slightly) beyond what they do?
Does it work better with the adaptive dome light? It requires light cache for GI and you won't need light portals.

[Druski]

Take the example attached. The scene is simple geometry, with a spherical HDRI projection, and VrayAreaLights (set to portal mode) over the light emitting areas. The problem is, I need to fire GI Rays in order for the portal lights to work, and that's a lot of tracing I would rather not be doing. Can the portal light mode be adapted to work for self-illuminated surfaces that do not emit GI rays?

[yolov]

Is the HDRI attached to a dome light? A recommended setup here would be to use it with a Dome light with the Dome Adaptive option enabled. You will need to enable GI and set the secondary bounces to Light Cache. You should also enable Adaptive Lights from Overrides > Lighting > Light evaluation.
With this setup you should not need portal lights and the Light Cache GI is very very fast.

[dgruwier]

yolov pretty sure the HDR is projected on geometry using a self illuminated material, meaning he can’t just use the dome light instead.

Druski
It's counter intuitive, but try enabling GI (default BF+LC), remove the portal lights, and just see what happens. Also assuming you're using a VRayLightMtl, try disabling Direct Illum.
Vray has become so good at using the light cache to optimize sampling that complex scenes can often render faster with GI enabled. I do a lot of work with HDR projected on geometry exactly like this, and just doing those things has sometimes cut the render speed in half or more.
If that doesn't work, you probably need to do some more manual work. Cut holes in the mesh and use adaptive domelights like Yolov suggest, or match the lights color manually to the HDR and use them as regular lights instead of portal lights, with the HDR clamped.
To answer your specific question, I don't know for sure but I don't think portal lights will work like that.