How to speed up rendering on low opacity objects

[boxfx]

One of our biggest struggles with render time is on any object which is mostly transparent. Imagine some sort of vacuum cleaner airflow type graphic, it could be some gradients and noise on a plane, or some gradients in the opacity of a hair object.

It seems to me that the samples an object receives is directly proportional to its opacity. If an object is 90% transparent, then it is only receiving 10% of the samples. This means to get a clean render on such objects, we have to increase the sample count several fold to get it looking clean. Compounding this is the generally very slow render speed of overlapping transparencies to begin with.

The attached scene is a visually very basic thing, but a 1000x500 pixel render on a geforce 3090 with nothing else in the scene takes 4 minutes. By the time the rest of the scene is introduced and we render at the full 4k res needed, we're looking at over an hour for a clean frame. For reference, without the transparent object, the frame renders in around 1 minute.

Does anyone have any advice on what we can do differently to speed up these renders? We use direct lighting for speed, adaptive rendering samples and have employed every speed trick we can think of.

project attached
c4d r25
octane 2021 latest

[champton]

I experimented with the scene for a bit but was experiencing the same problem on 2x 3090s. I think the best solution might be to try standard render + After Effects to get the look you're going for.

If someone could figure out a trick for this though, that would be awesome.