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.

Runtime memory issues probably have a significantly higher total cost.

But of course that doesn't negate this cost, but building can always be done on another machine as a way to circumvent the problem.