For rendering -- I hear a lot about Alias telling people to watch which shader they use, etc. for optimization. And I'm wondering -- what's the biggest time-drain?

Besides hardware particles (running a GeForce card) I'd guess that 2d motion blur sucks the most time out, but raytracing I'd assume would be pretty close.

I'm expecting a little deviation between answers -- but I'm still a bit curious... because I've been getting frames done with dynamic lighting, raytracing, ~50 shaders and have been getting only 2-5minute render times with 8-20x AA... I hear horror stories of 1-2 day renders -- so I know it can get a lot worse... but how?