Photon mapping is awesome, your box just sucks :P

The lighting in Halo 2 is far better than in Halo 1, so that just might explain why it takes longer. Also, when you're a company making tools for your own internal use (which is what the tools were originally made for), you don't worry about how long it's going to take on a mid-range PC to run lighting; you have high-end machines dedicated to that task.

Nick