Many software programs fail to achieve ultimate efficiency either because the software engineers are unable to do so, or because external factors prevent them from achieving it. I believe that in most cases, it is the latter.
IME the most underrated optimization tool is the delete command. People don't realize that it's something you should frequently do. Delete a function, file, or even a code base. Some things just need to be rewritten. Hell, most things I write are written several times. You do it for an essay or any writing, why is code different?
Yeah, we have "move fast and break things" but we also have "clean up, everybody do their share." If your manager is pushing you around, ignore them. Manage your manager. You clean your room don't you? If most people's code was a house it'd be infested with termites and mold. It's not healthy. It wants to die. Stop trying to resuscitate it and let it die. Give birth to something new and more beautiful.
In part I think managers are to blame because they don't have a good understanding but also engineers are to blame for enabling the behavior and not managing your managers (you need each other, but they need you more).
I'll even note that we jump into huge code bases all the time, especially when starting out. Rewriting is a great way to learn that code! (Be careful pushing upstream though and make sure you communicate!!!) Even if you never push it's often faster in the long run. Sure, you can duct tape shit together but patch work is patch work, not a long term solution (or even moderate).
And dear God, open source developers, take your issues seriously. I know there's a lot of dumb ones, but a lot of people are trying to help and wanting to contribute. Every issue isn't a mark of failure, it's a mark of success because people are using your work. If they're having a hard time understanding the documentation, that's okay, your docs can be improved. If they want to do something your program can't, that's okay and you can admit that and even ask for help (don't fucking tell them it does and move on. No one's code is perfect, and your ego is getting in the way of your ego. You think you're so smart you're preventing yourself from proving how smart you are or getting smarter!). Close stale likely resolved issues (with a message like "reopen if you still have issues") but dear god, don't just respond and close an issue right away. Your users aren't door to door salesmen or Jehovah's Witnesses. A little kindness goes a long way.