Actually, I've noticed the opposite in Poser 6: The P4 Renderer seems to be significantly faster than the Firefly Renderer. Here's an example:
I set a scene to render to the main window, at 400x600 pixels (72 dpi).
The P4 renderer (with the "Ignore Shader Trees" option checked) took about a minute or maybe less.
Using the Firefly Renderer at the third-highest quality setting took ages: I stopped it after 20 minutes of waiting for Poser to go through it's "Adding objects" and "Calculating lights" routine.
(And unlike many versions of Poser 5, the cancel button worked nicely: it stopped rendering and I could get back to work.)
 
I then changed the dimensions to 4800x7200 pixels, 200 dpi (for a large print),and the same third-highest quality using Firefly. I started rendering at 12:45pm Sunday afternoon. Poser was still rendering when I went to sleep at 11:50pm Monday night. When I woke up Tuesday morning, it was finished and I was able to save the rendered image.
Did I make a complex image with 20 characters? Did I have a million polygons? Nope, this is the image that took over two days to render: Against the Wall 2005.
Unfortunately, the maximum render size for the P4 Renderer is 4096x4096, so I have no choice but to use the Firefly Renderer for large images.

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