Well, what is the point in this comparison then? I'm not trying to say Rust is the fastest compiler on the planet, but really people don't even try to be objective.
If you're comparing, say, a 10 KLOC C++ program vs. a 10 KLOC Rust program, then the comparison is fair IMO. The end user doesn't care whether Rust chooses a different compilation strategy for dependencies.
End users run "cargo clean" really rare (I can't even name examples - really rare). If we compare regular compilation in C++, then we should take regular compilation in Rust, not something exceptional.
I have Rust projects that are many times smaller but take much more time for a non-incremental build. Of course, a large difference is that a fresh Rust build (after a cargo clean) compiles all its dependent crates, whereas many C/C++ libraries are provided pre-compiled by whatever system you are compiling on.