VRAY GPU Lightmix slowdown!

[mdm79]

Hello!

Im doing alot of retail renders and therefore love using Lightmix! to be able to control lights on the go in the VFB! I feel I have noticed an extreme slowdown with Vray gpu and using lightmix, It feels way to slow to use compared to the loss in render speed.

I have upgraded my system recently to a 5090 system. Running all the benchmarks everything is fine - Averaging the 15k in rtx benchmark! And GPU load 100% tracked with Gpu-Z!
I open a standard scene in 3dsmax 2025 with lightmix enabled, maybe 40 Unique lights in lightmix that needs to be controlled~ Mixed vray mesh lights, vray ies lights and regular vray lights (Plane, sphere etc.)
When I render I get extreme GPU load spikes - Instead of maybe between between 70-100~% I would expect? I go from as low as 1% to around 80% and render speeds drop immensely! - Final render time for the frame 20min~
WHen I render with Lightmix Disabled I get GPU load 100% and expected render speeds! - Final render time for the frame around 9min~
Is the Lightmix really expected to slow down this much with 40 light groups?
It happens in all my scenes using Lightmix!

Kind Regards

[Muhammed_Hamed]

Hi mdm79 and welcome to the forums!

Indeed if you have many render elements(including Light Mix) there will be a slowdown with V-Ray GPU. We are aware of this and we have a proof of concept that could improve the performance. Generally speaking using Light Mix elements in the standard V-Ray CPU(or other renderers) will affect the performance slightly, which is expected to happen. For V-Ray GPU the slowdown is extreme, rendering could be 2x slower if you add enough render elements.
My suggestion is to use bucket mode, which works better on GPU if you have many render elements. If possible try to use the least amount of render elements, until we find a proper solution.

[mdm79]

Hello Muhammed!
Thx for fast feedback!
Like your mentioning I also tested the scenario on CPU and found out the impact was alot less than GPU! So I thought there was something off~ For now Since the rtx 5090 GPU is so powerful, it seems I can adjust the lights in IPR mode and disable lightmix in full 2k resolution and get very fast feedback in mere seconds! That is "ok" for now but hopefully there will be a more proper solution for the Lightmix! I really feel the rtx 5090 has alot of power left when doing this - its crazy! Would have loved to have 2 cards like your setup, but getting hands on the Fe cards seems quite impossible! And money for the Motherboard, Oh boy! :O)

Kinds regards