1. For reviewers: Be aware that the files you choose to start a code review tend to receive more attention. We tend to get tired of reading code and have less energy (and attention) towards the end of the review.
2. For authors: Guide the reviewers toward the most challenging parts of your change. Leaving pull request (PR) comments on the most relevant lines has worked for me in the past.

There's a nice presentation about this with more details and data to back it up:

https://www.youtube.com/watch?v=_SJL7vepQvU

Alberto Bacchelli: How code review works (and doesn't) in the real world.