In the 90s I was given the task to make some changes to a shell script which provided a complete systems-engineer configuration management tool for country scale network management systems.

This shell script was 9,000 lines long. Each new "menu" after entering a number corresponding to your selection from a previous menu took 60-120 seconds to appear. Needless to say, configuring systems was a painfully slow process.

I quickly found it practically impossible to extend this tool to meet the new requirements, so I quietly rewrote it (also still in shell script, because of host limitations). The new version was 1,000 lines, and menu changes took < 2 seconds in most cases (thanks to appropriately placed caching).

Management was not happy that I decided to rewrite (since the manager had written it himself originally), but the users were ecstatic about the performance increase. I would not be surprised if that tool was still in use today.