The argument could be made that compile-time processing is highly appropriate for the static inputs of a program, and, at the other extreme, obviously can't be applied to the dynamic inputs. (Terminology from https://en.wikipedia.org/wiki/Partial_evaluation)

Of course, the raytracer source code is the dynamic input of a compiler. The Futamura projection perspective suggests interesting options here for e.g. specializing raytracers in relation to particular scenes.

C++ compilation is massively slowed down by the sheer existence of the preprocessor. This could be drastically improved by the usage of modules which are coming soon(tm).

Both MSVC and Clang already support them experimentally fwiw.

To be fair, what slows down compilation is not "the sheer existence" but the extensive use of large and complex #include files.

Indeed. IIRC this topic is covered directly in Meyers' "Effective C++" book, including the trick to use pointers + forward declarations in headers to avoid including other headers.