The same way it always happens to me: Over time build time creeps up and you don't really look at this code thinking "I implemented shallow clone years ago so there is nothing I can do, it's slower because we have more code."

Until you or some other person looks what the code is doing.

It could also be that it was a new hire. I shallow clone a huge monorepo similar in commits/branches and it takes seconds. My experience would instantly tell me that something is worth looking into.