You're correct- what I had meant to say is Rust is smarter with more runtime checks over C for several cases, like RefCell. Therefor, Rust can be slower than C because of safety (but not because of type checking itself).
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.