Using the latest DAZ Studio gives you a substantial iRay speed boost, thanks to the integration of the latest iRay version. 2x faster on CPUs, for instance. Though, as NVIDIA want to sell $500 graphics cards, you’ll only read about that in a quiet blog post from the iRay dev team.
But now I read on the forums about another way to add a further 2x boost to DAZ Studio. So long as you don’t have transparency in the scene, choosing the “Interactive” rather than “Photoreal” version of iRay in DAZ Studio should apparently give you another 2x speed-boost with minimal loss of quality. Some shadows are skipped and transparency is handled differently. But otherwise it’s a good WYSIWYG representation of what your render will look like. The advice is from someone on a fast dual-Xeon PC, but it’s something to consider re: working toward a real-time iRay viewport rather than just a Preview window. It’s easily tested on your system.