Mary's composite shadows look a bit different than Candy's in the same spot, so you might have slightly different lighting going on too, but I'm guessing it's close enough to look right in the composed shot anyway.
If the composites are rendering quickly but coming out grainy, there's a few tricks you can use:
- Make sure the rendering converged ratio is set really high (95% is the daz default, but 99.9 makes sense in an image where 95% of your pixels are blank)
- Bump up the min-samples (for the office, this is my go-to trick)
- Double the resolution, then shrink the image by 50%. Wasteful but effective
- Set the spot-render tool to render in a window, and select the part that came out grainy. Then photoshop that back into the original grainy image. Don't ask me why that works better than the converged ratio trick. Used this one a lot in the inn.