Casting Shadows, may be pertinant for this render, but is intensive.  I never want shadows, and always keep it off; becasue it's a hog, and am always trying to aviod shadows anyhow.

The RayTrace value is how many times a ray will bounce, and receord, between two mirrors.  This scene is especially effected by Ray TRacing, but the manual reccomends 3 to start with.

Irridesence has to do with light levels, on objects.  If you lower the value you become more accurate, but add render time, and vs. versa.

I doubt you're using DIssplacement, and it's kinda intenseive.