Daz Daz only using RAM and CPU

[Powerline75]

How come Iray preview takes so long to produce a picture and yet octane can produce it in a couple of seconds

In short, unless you manually set the shaders for each surface, you render "flat" (flat as in non shaded, other than diffuse texture, and, maybe, bump, normal and specular maps) surfaces in octane. And, as in any render engine, CPU and GPU alike, flat surfaces render way faster than shaded ones. Iray, especially when ran through DAZ, previews everything with DAZ optimized shaders, and the GPU runs ALL the math for each shader. Yes, math. In their core, shaders are nothing more than logic gate configurations, that tell the GPU how to treat each pixel, when it gets rendered.
Octane, on the other hand, doesn't recognise those shaders, and mostly goes with diffuse, bump, normal and/or specular maps, as a generic, universal shading node. And, the lack of specific instructions per material zone, surface and face, makes up for the speed difference in preview, and final render alike.