I'm actually working on a movie and despite of lots of optimisations, renders are very long for dark reasons.
The shader is very simple, and there are only 3 or 4 textures in the scene (that I converted in Tx).
But has Stephen said, among the log, this is not a texture problem.
Wich is very stange is that we have change the desing of the object during the production.
The old one (Quartz) was much more complicate (complex shaders and geometry / thousand of objects / complex refraction / displace ...) but faster to compute.
The new one is simple, but very, very long to compute.
I behave that on a pure color shader, noise is more visible and more difficult to get ride of.
But on this exemple, I jumped from 39min to 1h20 by changing the design with the same render parameters.
After ajusting them for reducing noise, it take 2h30.
As you can see on the third picture, I effectively Increase Transmission and SSS samples, but deactivate GI (0 ray in second bounce of diffuse and spec), and reduce transmission depth from 5 to 3. :
Moreover, I use RaySwitch shader in each material of the scene, with a pure diffuse material for the switch and exclude other objects from some rays :
I launch 15 differents renders, with 15 differents combinations of parameters and nothing allow me to raise acceptable time with acceptable noise.
Here is the Log :
