You have no valid comparison. We have no way of knowing whether the materials are comparable, nor whether the render settings are comparable. I'm pretty sure that iRay isn't twice as fast as Cycles, though. You may have applied redundant samples/bounces in some category(s), and/or used a tile size which is not a common denominator of the image height/width.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5