Here are a couple of options.
You can recreate the shape of whatever room you are using with cube primitives, using a low number of subdivsions, and then use simple shaders or just simply color the cubes. Of course, you'll scale the x, y, and z parameters of your cubes appropriately so that it's more wall shaped and less box shaped. By lowering the number of subdivisions in this way, it can help a small bit with the Iray math, as there's less 'sub surfaces' to reflect off of.
Planes are good for this too, and have less faces, but I like cubes as they give you more edge/surface options, plus you can assign different faces to different texture groups using the Geometry Editor tool in Daz or sending the cube to Hexagon to do things like adding a simple doorway or a window. Just make sure your cube has JUST enough subdivsions where you can delete a couple of faces to create your 'hole' and then use the bridge tool in Hexagon to create the 'inner edges' of said window or doorway. You can also texture interior and exterior walls on the cube separately with different texture groups, say if you want to put two rooms next to each other and have camera shots in both rooms that look into the other room.
Another thing you can TRY is to take an asset piece into Hexagon and decimate it, reducing the number of faces. This can mess with textures though, so you may need to re-map those using the cubic, planar, etc. mapping feature in Hexagon. Decimating can also change the 'shape' of an object depending on which vertices end up getting removed.
You can use Blender too. The nice thing about Hexagon is that it bridges directly with Daz3D, so sending objects back and forth using Hexagon may be a bit more convenient. Hexagon likes to crash occasionally, though, so save your work!
Blender is an awesome program though, so if you can get the hang of it, well it's definitely an option.
Others already mentioned scene optimizer. Shaders are also an interesting option. Sometimes you can just re-texture a wall with a simple shader, one that may or may not have an associated texture map, to simplify the wall/reduce the wall texture size.
I use shaders often to 'refurbish' old Daz assets. The older Daz assets were generally built using lower spec machines, so they can be a bit less resource intensive depending on the assets. Of course, the texture maps are more primitive as well, but modern Iray shaders can really spruce things up. For clothing items as well as walls, etc. assuming you can get the clothing item to autofit reasonably well...
This brings up another interesting point. Sometimes, you can swap out the texture maps for an item of clothing using simpler cloth shaders, which may help on your scene overhead. I do this often enough, especially if I want a different pattern on a shirt, etc. or just want simple color groups.
There ARE indoor HDRIs out there, available for free. These can be a bit more tricky to use, but for 'simpler' scenes, well the HDRI doubles as your lighting source, and can end up with some pretty compelling results, even with 'just' 4K HDRI maps, which I use the most often.
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Here's a shot from Enthralled that I did, that mostly uses a HDRI, although I had to add a couple of planes/cubes to 'hide' parts on the left side of the HDRI and the floor that I didn't want to appear in the shot, texturing said wall and floor with suitable shader textures.
Not perfect, but it was good enough for my purposes. Anyways, my point here is to keep your options open, sometimes you can simplify things and still get a good result.
Hope this helps!