I see this getting computed the most when we are rendering particles which have emission on them.
For example, when I turn light tree on - it took a frame 1 min 14 seconds to render. And turning off light tree made the render time 4 seconds.
And another question for blender particles:
Basically, the premise of this research is how to make particle process (set up and bake) fast and render efficient. Any tips on the same would be greatly appreciated. 
Hi,
I think when it says particle group or whatever, means that you would need to render exactly those objects that are appear in your camera field view. I have found something about this here:
Hope it would help you anyway
As for the object group, I’m not sure, but maybe the author tried to say that you need to put your objects into a collection, but I have no clue how it could help to reduce the actual render time. But from what I know, you can use so-called Instance Collections. Means, if you have one object and you want to make a duplicate of it, let’s say, 4-5 times, you won’t have to just literally copy the original one by pressing Shift-D, because it will increase your polygons’ count at the same time. Instead of this, you can easily use Instance Collections, it works basically the same way, but here you will need to press Alt+D instead of Shift+D. The only thing about that is when you make an instance of your original object, you won’t be able to bring some different changes to that exact copied object, you be able to work only with the parent one, thus, no matter what would you do with your parent object, all instances will automatically take those changes on them, and at the same time you won’t be able to bring some changes to your instances ones. Hope I’ve explained that correctly 
Hope that helps
Cheers,
Sergey
The light tree helps reduce the noise when a render has multiple light sources. It will help Cycles choose which lights needs to be sampled for each part of the scene instead of wasting rays on lights that are far away or hidden.
If you were to render, let’s say, a Christmas scene with hundreds of lights everywhere, the light tree would help a lot and will make the noise clear in a reasonable amount of time. However, if you render a scene with just one or 2 lights, it might be better to turn it off, because it does have a performance cost.
It also produce a lot artefacts like hard light shadows, even on area lights.
I have never seen such artifacts from light tree. Are you sure it’s causing them (like, they disappear if you disable light tree)?
Yes, I have them a lot. And yes they disappear, if I turn it off.
Here is a sample image. Note there are no hard lights in the scene that could cause that lines.
I just tried on and off in one of my scenes and the only difference is that light tree is less noisy. This looks like it might be a bug happening with your computer specs.
If you try it on CPU, does it still do it? CPU rendering tends to be more reliable and predictable, so it’s likely that it wouldn’t do it if it’s a system related bug.
Forgot to mention, it’s also GPU related. Interestingly it affects all GPU, Nvidia, AMD and Silicon.
But who is still rendering CPU ?
My GPU doesn’t have any problem in my own scenes.
Would you be willing to upload a scene where you have problems? If you don’t want to upload a full project, you could just do a single object and its light source, the smallest scene that causes you problems. Just to make sure it’s truly system related and there isn’t actually something in the scene itself.
Yeah I have too… it’s on my list. I just have not the time for it.
