I just got Poser 5 about a weeek ago as a birthday gift for myself. I had been planning to keep using Poser Pro as my primary program and just go to Poser 5 toward the end of a scene for shaders, dynamic cloth if needed, and a lot of cursing. But when I tried P5, you may think I'm smoking crack, but it was faster. A lot faster! Not rendering, but loading up and bringing in figures and stuff. And if I'm just doing a test render, all special features turned off, Firefly seems to take around the same time. (Admittedly, I multi-task like an octopus with an attention deficit, so maybe I'm just not noticing the difference. And more final renders with the options on does take longer, but I'm OK with that.)
I'm on an XP box with 448 MB ram, and I did a nice neat install as suggested in a stickied thread at Poser Pros. Install P5, then install the latest SR and remove Content Paradise before opening, then properly install all the content I want to use instead of just copying over my P4 runtime.