Ask HN: How do you give thoughtful reviews on code you've never seen before?

I am frequently tagged on code reviews for large (2000+ lines) changesets to portions of a large, complex codebase that I haven't even seen before, just because it's "frontend" and I am one of the frontend developers, and am supposedly therefore qualified by default to critique the code.

I find this very challenging, and sometimes spend long periods trying to figure out what the context is, what the existing code did, what the goals of the changes were, and how the author went about implementing those goals. This can easily take 3-4 hours or more depending on the code, if I'm saying anything more than "missed a semi-colon there." And often can result in a simple "looks good to me!," leaving me feeling like I wasted an afternoon.

How do you (or your company as a whole) deal with this problem?