VrayColor2Bump in VrayBlendMtl doubles rendertime

[racoonart]

I'm currently having problems with VrayColor2Bump nodes in VrayBlend materials.
To follow along, please have a look at the images below and the rendertimes:

00_setup (next post):
how the materials are setup

01_Layered_Complete_2m25s:
That's the complete material, 3 layers with 2 masks. An outer plaster material, an inner damaged plaster layer and last but not least the bricks. Each of the materials is using a VrayColor2Bump node for the bump details.

02_Layered_Complete_NO_BUMP_1m07s:
The rendertime drops to less than half when I just disable the bump channel on each material

03_Layered_Complete_NO_BUMP_BLACKMASKS_1m01s:
The rendertime drops a bit more when I make all the masks black, effectively only rendering the outer layer

04_Layered_Complete_BLACKMASKS_1m51s:
Now I enable the bump slots again but leave the masks black (effectively the same image). Notice the massive increase in rendertime again

05_OuterMaterial_only_1m06s:
for reference, a render of just the outer material, nearly similar to 03 but with bump.

Conclusion:
Disabling or enabling layers (hidden or not) doesn't affect the rendertimes much... if at all. However, rendering with bump maps increases rendertimes considerably - no matter if you can see the individual layers or not.
I can provide the scene but I can't upload it publicly, please let me know where I can drop a file or a link to the file.

V-Ray Next, update 2.1 (4.20.01.29465)

[racoonart]

the setup (couldn't upload it the first post due to 5 item restriction)

[^Lele^]

VRayColor2Bump is expensive, if very pretty.
What happens if you use a simple bitmap in the bump slot instead?

[kosso_olli]

VRayColor2Bump is expensive, if very pretty.

I never understood what it does and when it is needed. Could somebody please clarify?

[racoonart]

VRayColor2Bump is expensive, if very pretty.
What happens if you use a simple bitmap in the bump slot instead?

If I use an arbitrary bitmaptexture (with a hopefully similar enough noisy structure) on all 3 materials, the render times go up to 1m38s.
If I leave the outer material with the original VrayColor2Bump (but the inner ones with the bitmap), the rendertime goes up to 2m04s.
My guess is that both materials in the layers below will be fully evaluated, despite being occluded.

The problem is that I seem to need VrayColor2Bump to get any bump at all with OSL textures. If I just plug in the osl map directly into the VrayMtl it's extremely weak (if present at all, not sure). I wanted to investigate that further and write another report but I'm already way behind schedule and everything's on fire aleady

[edit]: nope, bump works, but is just a bit less pronounced it seems, my bad

[edit2]: now, with all original maps directly plugged into the bump slots, rendertime is at 1m40s. Not quite as low as I had hoped but at least better. Unfortunately it also loses the "depth" it had before, no matter how much I increase the bump value. See the image below.

[^Lele^]

My guess is that both materials in the layers below will be fully evaluated, despite being occluded.

Correct. This seemingly can be optimised (so i have been told. ).
Thanks ever so much for putting more time into it.

As for the vraycolor2bump map, it evaluates the texture more than once, unlike standard bump.