If a block of code is only used within a single function, the block of code should reside within that function rather than in a separate subtroutine, external routine, or external function. The #OPTIMIZE compiler command works more effectively when optimizing code within a single function. The actual code execution (net of PSL) is significantly faster if the code is adjacent, rather than in a separate label or program.
If there are multiple references to the code, the benefits of reusability and maintainability outweigh this performance benefit, and the code should therefore reside in a separate label.