If you're writing your program in C, you're afraid of shooting yourself in the foot and introducing security vulnerabilities, so you'll naturally tend to avoid significant refactorings or complicated multithreading unless necessary. If you have Rust's memory safety guarantees, Go's channels and lightweight goroutines, or the access to a test runner from either of those languages, that's suddenly a lot less of a problem.
The compiler guarantees you get won't hurt either. Just to give a simple example, if your Rust function receives an immutable reference to a struct, it can rely on the fact that a member of that struct won't magically be mutated by a call to some random function through spooky action at a distance. It can just keep it on the stack / in a callee-saved register instead of fetching it from memory at every loop iteration, if that's more optimal.
Then there's the easy access to package ecosystems and extensive standard libraries. If there's a super popular do_foo package, you can almost guarantee that it was a bottleneck for somebody at some point, so it's probably optimized to hell and back. It's certainly more optimized than your simple 10-line do_foo function that you would have written in C, because that's easier than dealing with yet another third-party library and whatever build system it uses.