Curios to see my post pop to the fore but I know well this is an ongoing issue. I will add what I have learned in the interim because while I still struggle with this I have come up with a few strategies that can ameliorate the problem.

• The geometry of the model and textures take up memory on your care. The more complex or larger these ore for either the greater chance your render will no longer fit into you GPU's memory and you will be relegated to the CPU. You can reduce geometry and textures but you also lose quality. The best answer is to get a bigger GPU video card with lots of memory like the GTX 3090, oh wait ;)

• Depending on you rig you card can start getting thermal checks. I use TechPowerUp to monitor for this and coupled a little fan I purloined from my daughter I have found a constant stream of air in addition top case fans can speed up renders. Of course I water cooled GPU would help but see item one for resolving that issue.

• Lights, the more complex you lighting the more intensive the calculation will be to complete your render. It's often a tricky balance between quality and speed here.

• Closed raytracing. I have found experimentally that rendering inside a close box can take longer. If you allow your ray traces to escape to infinity it appear that Iray will kill them off rather than doing all the reflection calculations. Your mileage may vary here.

That's what I have off the top of my head, oh wait another alert about an RTX 3090. Gotta go.