That isn't always the case. Slow compilations are usually because of procedural macros and/or heavy use of generics. And even then compile times are often comparable to languages like typescript and scala.
Compared to C, rust compiles much slower. This might not matter on performant systems, but when resources are constrained you definitely notice it.
And if the whole world is rewritten in Rust, this will have a non-significant impact on the total build time of a bunch of projects.
It's been mentioned obliquely, but what's considered "part of the compilation process" is different between Rust and C.
Most of the examples of "this is what makes Rust compilation slow" are code generation related; for example, a Rust proc_macro making compilation slow would be equivalent to C building a code generator (for schemas? IDLs?), running it, then compiling the output along with its user.
Rust compilation is also almost instant, if you have a relatively small project.
It's only when you get very large projects that compilation time becomes a problem.
If typescript compilation speed wasn't a problem, then why would Microsoft put resources into rewriting the tyescript compiler in Go to make it faster[1]?
That isn't always the case. Slow compilations are usually because of procedural macros and/or heavy use of generics. And even then compile times are often comparable to languages like typescript and scala.