Oddly enough, I asked this same thing earlier over at Daz..this is what was told to me that can slow down renders.

• IBL with many lights (high quality),
• with IBL, transparent materials really slows things down,
• many light sources,
• soft shadows,
• high frequency procedural materials,
• Premium render options,
• volume materials,

not sure if that helps you any..but be patient..lots of helpful folks around here as well :o)