Interesting real-world case-study over at Smashing Magazine:
In this article, we’ll take a close look at some of the changes we made on this very site — running on JAMStack with React — to optimize the web performance and improve the Core Web Vitals metrics. With some of the mistakes we’ve made, and some of the unexpected changes that helped boost all the metrics across the board.
Be sure to check out the linked diagnostics.css by Tim Kaldec. It can be used to indicate clear (or potential) problems with the markup.