When referring to the critical rendering path in site performance, a critical resource is:
a resource that could block initial display of the page
any resource that returns 404
any resource that must be loaded
a resource that takes more than 500ms to load
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: //support.google.com/partners/answer/7336426