Yup. If I'm working with code I can understand, making changes takes very little time. If I'm working with code I don't understand, I might be able to hack some

Yup. If I'm working with code I can understand, making changes takes very little time. If I'm working with code I don't understand, I might be able to hack something together that makes things even harder to understand. Probably the best bet is to spend however long it takes to dive into it, and possibly rage-refactor if it's too much to dig into without being the person who wrote it.