The question would be for me, can we run those slow-rustc cases faster by skipping the part corresponding to "external checker program"? Or is that part required for each and every build, even after tiny changes or no changes at all?
It's not that relevant though, because the longest part of any Rust build is the dependencies, and those are never recompiled. Most projects compile relatively fast once you've built them once.
When you do full comparison of what rustc does, and compare it to what you need to do in C++ project to match it, the compile times aren't actually slow since rustc does more with actual guarantees.
Having some additional features and checkers is nice, and can even be a life saver, but having to pay a high cost each time I hit compile is not acceptable.
So the compiler will spend some time inferring the types of different variables and what traits those types implement, which is necessary to know which trait methods get called, etc.
Dynamically-typed languages get to skip all this and say "my type is either a number or an object, because everything in the universe is either a number or an object", and methods are selected through dynamic dispatch (usually by storing a ton of function pointers everywhere).
Rust can't just do dynamic typing and skip the compile-time logic, though you can get close by using lots of dyn traits.