Use light cache for glossy rays is great way to reduce render time, but sometimes we have artefacts with it.

There is simple way to avoid it, it is to turn Off prefilter and On Filter Fixed in Light Cache Reconstruction_parameters with value if Filter Size 2-3 times bigger then Samples Size of Light cache.

VrayLight in portal mode MUST have put on the outside of the room, and not internal, as is the normal practice. The noise will be much less. Well, and of course subdivs for portal mode (which means primary light will be either Sky or HDRI) will need two times greater than for normal VrayLight of the same size.

The speed/quality can be adjusted using only 4 parameters:

1. noise threshold in DMC
2. global subdivs mult
3. subdivs at LC (in new VrayC4D it`s can be auto adjusted too)
4. resolution
The rest is not necessarily touch. Auto changes. This applies to all settings Ir map and DMC GI.

And yet! Use DMC AA 1/4. No Fixed!

(c) Saluto

Noise is different and treated differently.
Only describe the causes of small grains (no stains!).

1. Noise directly from the light. Often, on the border of light and shadow. Particularly strong from Vraylight if there is an object. The closer the object is to VrayLight, the more noise. Increase subdivs of Light or AA settings (preferably find averages for both, not only raising a one parameter)

2. The noise in the materials (and reflect refract glossiness). It depends on the settings of glossiness subdivs in material. Treated with settings of material and AA. Again, it is desirable to gradually raise both. If speed is important of course. If not, you can use DMC AA and increase global multiplier and noise threshold.

3. Noise from highlight glossiness. And often it is not important, use you this option in the material or not. Just when it is used appears more noise. So, the noise treated with rising subdivs but only at VrayLight (NOT IN MATERIAL!) And the AA settings. Naturally you must raise subdivs in that Light from which is the noise.
(c) Saluto