Things can still be slow in absolute terms without being as slow as C++. The issues with compiling C++ are incredibly well understood and documented. It is one of the worst languages on earth for compile times. Rust doesn’t share those language level issues, so the expectations are understandably higher.
Rust shares pretty much every language-level issue C++ has with compile times, no? Monomorphization explosion, turing-complete compile time macros, complex type system.
There's a lot of overlap, but not that simple. Unless you also discount C issues that C++ inherits. Even then, there's subtleties and differences between the two that matter.