How can you tell if it aligns with the goals of the project without reviewing 9000 lines of code first?

PRs rarely exist in a vacuum. Usually there is a ticket/issue/context which required a code change.

Read the title and description of the PR first. If that fails the sniff test, the code would as well.

Are you kidding me? You should be able to explain from the user PoV what does the PR achieve, a new feature? a bugfix?

That data point is waaaaaay more important than any other when considering if you should think about reviewing it or not.

Okay, it does align. What next?

Ok great. Now that it aligns, how valuable it is? Is it a deadly & terrific feature? Then maybe you should review it. It's just fixing a nitpick? Then ask the contributor to find a less verbose change.