The fastest script in the world is worthless if a change in your
system's directory structure breaks your code and you can't fix it.
If you bother to optimize for anything, do it for maintainability
But never forget "monitorability".
Unless your script is being called to do huge jobs, or your resources
are very restricted (Sparc Ultra 1 or Intel 486, etc.) optimization for
speed is not usually that big an issue.
However, thorough and correct logging of events, meaningful commentary
in the script itself, reusability of the code; these will all help