Iterative C++ compile times can be very fast if you know what are you doing, 17 seconds is very long unless you have link time optimizations on(which makes no sense for an iterative build).
As far as I can tell they didn't change a shader at all, they linked to code that looks like it runs on the CPU as part of the main binary not as a shader.
Please remember there's a lot of optimizations put in the legacy compilers.
What makes you think Rust compile times won't improve?