An extra step doesn't sound like much, but it has considerable consequences. This trade-off for easily sharing ownership across various functions or threads removes the ability to put as much data as possible into the CPU's cache, since the pointer makes any data locality difficult. Heap allocations themselves are expensive operations and reducing those will already provide a major speedup.
Furthermore, the compiler cannot deallocate a boxed value if it's still referenced in some places—a problem that occurs especially if the program is large and complex. Similar to orphaned objects in C# or Java, a saved Rc reference can easily be forgotten, creating a memory leak. Therefore, it's recommended to use heap memory only when required.
One principle piece of advice that requires a boxed value in Rust is to "favor object composition over class inheritance" (Gang of Four 1995:20). In the absence of class inheritance, the choice is obviously...
