As noted in other comments, this was a case where the pedagogical goals of the system outweighed maximizing performance.
It's really incomplete, and I have had hardly any time to work on it over the many years that it's been around, but the general idea is to type construction to determine the aspects that a scene actually uses and let the compiler sort out what to do about it.
For example, the depth count attached to a ray doesn't appear until it hits a reflective surface so there is no overhead if the scene doesn't need it. As you can probably imagine the template errors can be obscene :)
The next thing would be to constexpr everything so that the scene could be built up in the compiler, I guess ideally in parts and then combined by the linker. I think it'll be really fun to try that with this Moana scene, but don't expect the compilers or linkers will survive let alone the tracer
EDIT to say, I really love your book. I feel hugely inspired every time I look at it, which unfortunately means I can't look too often as I don't have the time to play with these things as much as I'd like.