When doing cloth sims in Poser, I always restart the software, and if I have been working for a while I may even do a reboot before starting. Also I do not apply any textures beforehand to the character or the cloth no need to clog the memory for nothing, I apply all the mat files when the sim is finished. I have also found that Poser is very fussy about the mesh for cloth simulations,I don't know if anybody can confirm this for me but I think that having many material zones makes the sim take longer.

Dominique Digital Cats Media