You could try DAZ Studio, which is still free, as far as I know.   I wouldn't expect that to be significantly better (or better at all, actually) than Firefly, though, although it may be faster.   You can import Poser files into Vue or Carrara, although neither will entirely faithfully replicate Poser materials at present (Vue 6 should fix that, but, now that Carrara is owned by DAZ, I'd expect to see Carrara get less Poser-friendly, rather than more).   In my experience, all three can get overloaded by a complex file  (D/S can, too).   Vue 5 Infinite handles memory problems much better than Poser or Carrara.   Carrara seems to slow down on hair (but does a very good job of rendering it).   Vue gets glacial when rendering water, but, again, for my money, nothing touches Vue for renders involving water.   It's also possible that you have Poser settings that are more demanding than are really required.