You're refactoring your code to improve performance and notice a function that returns another function, accessing variables outside of its own scope. What specific JavaScript concept should you consider when optimizing this code section?

When optimizing code that returns a function and accesses variables outside of its own scope, you should consider the concept of function closures. Function closures allow an inner function to "remember" and access variables from its containing (parent) function's scope even after the parent function has finished executing. This can be useful for encapsulating state and optimizing code.