How fast P5 can render is a bit of a "how-long-is-a-piece-of-string?" question, since everything depends on what is in the scene (number of figures, size of textures, number of lights etc.), plus which renderer is used, plus which render settings are used.   There were discussions here when P6 came out as to whether P5 Firefly or P6 Firefly produced the better results (with some feeling that P5 did), although  I have always preferred P6, ever since I located a  driver to get it working.

I wouldn't claim to have produced any  extremely realistic  renders ever, but they are possible with P5.      I'd recommend a look at the products by  face_off, depending on which characters you are planning to use.

Oh, and don't worry about the "breath of fresh Mustard Gas" greeting from Miss Nancy.   That's more or less traditional around here ;¬) .