A large number of ray trace bounces. Consider how many bounces to get the reflections you want, and consider a reflection map to fake it. 
Image size. Do you really need that pic at 6000 x 4000 pixels or would 3000 x 2000 do just as well.

I am not clear on the impact of large image maps on render times. Does a pic need a 3000 x 3000 texture for a character in the background thats maybe 100 x 100 pixels in the final render ?  The textures take up memory , and theres a limit on how much poser can allocate.