If you're just using Arc<T> without any other parallelism primitives, then it's immutable and the cores can all read without blocking. The only thing it does is

If you're just using Arc<T> without any other parallelism primitives, then it's immutable and the cores can all read without blocking. The only thing it does is reference counting to know when to Drop. Blindly using Arc<Mutex<T>> without considering access patterns is a software architecture problem, not a problem with Arc or Mutex.