Hi drifterlee, I used Poser too for a very long time, but a physical renderer like iray or superfly cant be compared with a renderer like firefly. Ok you can change a lot in the settings to get less grainy images and render the images faster. Dont forget that if you change some settings it can also take longer!. The first thing you want to do is, make sure you have all the materials on the models Iray ready, if not, then it can take hours because Iray simply don't know what to do with the render materials. Second thing i recommend is this:

a lot of users don't know that they can scale the 2 Environment Spheres , it will affect the light, and also the HDRI maps usually I use Finite Sphere with ground on scaled down to only 9% for portraits or close ups and use Infinite Sphere only for landscape rendering like nature or cities . I also highly recommend to check off the Burn Highlights per component , set the Burn Highlights to 0.25 set Crush blacks to 0.20 setting off the Crush Blacks to 0 will create less contrast it is good for very close up portraits.

Third:

The best is to use the Sun Sky with the correct day time latitude , I use often the Exif data info from my photos to get exactly the light and second options are HDRI maps there is plenty free maps online to download when you load HDRI maps use the Finite Sphere and scale it 9 %down when use with models but not always and the last but not least are photo metric lights in Daz Studio to use with Iray , do not used distance light or spot point light from DS, for PBR only photometric lights works best.

In this preview render i made everything myself( textures, characters etc). This is just the iray view and not even really rendered, as you can see no grainy bits. and it was done in 5 mins. I can rotate and change it in real time and i have a lot of less resources than you! This is due to the fact every skin map is iray ready, this way it will render a lot faster. Also if you have displacement maps, it will take iray longer to render the files.