The code examples are confusing. The show the code that takes the locks, but they don’t show any of the data structures involved. The rwlock variant clones the Arc (makes sense), but the mutex variant does not (is it hidden inside inner.get)?

In any case, optimizing this well would require a lot more knowledge of what’s going on under the hood. What are the keys? Can the entire map be split into several maps? Can a reader hold the rwlock across multiple lookups? Is a data structure using something like RCU an option?