- a resource that could block initial display of the page
- any resource that must be loaded
- a resource that takes more than 500ms to load
- any resource that returns 404
Explanation: A critical resource is a resource that could block initial rendering of the page. The fewer of these resources, the less work for the browser, the CPU, and other resources.
Read more here: https://support.google.com/partners/answer/7336426