The diff is in an area I do not know. What do I do?
Say so and review what you can: tests, error handling, whether the description matches the change. An approval that means "I did not understand this" is worse than no review, and a question in the thread is often more useful than the approval would have been.
Big PRs sit for days. How do we fix that?
Make them smaller, because attention does not scale with size: a 50 line change gets real review, a 2000 line change gets a rubber stamp. Splitting by mechanical versus meaningful is the easiest cut, since the mechanical half needs eyes but not thought.
How do I review without turning it into style comments?
Automate style entirely, then the review can only be about substance. What remains worth saying: this breaks under concurrency, this loses an error, this name means something else in our codebase. If a comment could be a lint rule, make it one.
Any downside worth knowing before I commit?
It commits you to a format that is tedious to migrate away from later. The first weeks also look worse than doing nothing, which is when most people abandon it.