You raise a great point here:

    > since I didn't write the code, in order to understand it I have to read it. But gaining understanding that way takes longer than writing it myself does.

I remember reading Joel Spolsky's blog 25 years ago, and he wrote something like: "It is harder to read code than to write code." I was quite young at that stage in my programming journey, but I remember being simultaneously surprised and relieved -- to know that reading code was so damn hard! I kept thinking if I just worked harder at reading code that eventually it would be as easy as writing code.