RefCell has an internal semaphore. It's used specifically for multi threaded scenarios. If someone is writing a multi threaded C app, they will likely be using

RefCell has an internal semaphore. It's used specifically for multi threaded scenarios. If someone is writing a multi threaded C app, they will likely be using semaphores as well. At least, they should be. Rust just enforces it. So, I wouldn't say rust is "slower" in the regard.