It could be the volumetric/media that is slowing down reflections, really clean volumetric media can be pretty time consuming. Still, this is pretty slow, maybe there's some excessive sampling somewhere that is not exposed anywhere and just fails to be efficient in this special case.
A honest question - did you consider other 3d apps for this kind of setups? I sometimes see scenes that would be much less a headache to produce in other 3ds apps. TG has some really cumbersome or extremely limited workflows for scene assembly/management, objects, scattering, manipulation, materials, animation etc. I wouldn't even think about using TG for most stuff, I personally think it's best suited for landscapes/nature, atmosphere and planets, and it can already be pretty cumbersome to work with in these cases. Not to sound like a smart*** but I think a different app would really be a better tool in this case.
Still, it's good someone checks the depths of PT in TG, it can be probably improved to be more efficient in other scenarios than what people typically do in TG.