FL, when rendering in Iray and using Interactive you can still use different lighting, scene lights and or HDRIs you aren't just restricted to using the Sun-Sky choice.
Interactive is a more basic render you need to fiddle with the different settings to get good shadows and occlusion, it might be a bit quicker but doesn't give as nice results, in my experience it takes so much fiddling for some renders to get a good result you might as well use Photoreal mode on my machine (CPU only) a portrait render will take about 3-6 hours depending on lighting and surface settings and size, that's using Photoreal mode.
Making sure that surfaces are converted to an Iray shader (Daz supplied an Uber Iray shader and some presets) will help speed up renders, if you don't convert Iray has to take the time to do it. Lighting is important for speed as well, a dark image will take forever a well lite image is much quicker, images that are inside take a long long time as well, I tend to use HDRIs and 1 light, usually a spot light or just a HDRI (the better the HDRI the better and quicker the results). Indoor images I use 3dl, night images 3dl. I have had Iray images take 15 hours any longer isn't really possible because I can't do much else on my computer when rendering in Iray.
Interactive is a more basic render you need to fiddle with the different settings to get good shadows and occlusion, it might be a bit quicker but doesn't give as nice results, in my experience it takes so much fiddling for some renders to get a good result you might as well use Photoreal mode on my machine (CPU only) a portrait render will take about 3-6 hours depending on lighting and surface settings and size, that's using Photoreal mode.
Making sure that surfaces are converted to an Iray shader (Daz supplied an Uber Iray shader and some presets) will help speed up renders, if you don't convert Iray has to take the time to do it. Lighting is important for speed as well, a dark image will take forever a well lite image is much quicker, images that are inside take a long long time as well, I tend to use HDRIs and 1 light, usually a spot light or just a HDRI (the better the HDRI the better and quicker the results). Indoor images I use 3dl, night images 3dl. I have had Iray images take 15 hours any longer isn't really possible because I can't do much else on my computer when rendering in Iray.