Note: For further information on Google PageSpeed Insights and why you should not bother chasing a "grade" please read this guide:
Google PageSpeed Grade does not Improve
Therefore it is recommended to delay (“defer”) the loading of non-critical files until the browser is done rendering the page. This requires adjustments to the HTML source of a page itself, as well as intimate knowledge of which file is required at what point during the rendering process. Sounds tricky? It sure is!
Nonetheless, tools like Google PageSpeed Insights makes it sound so easy:
Like many of PageSpeed Insights’ suggestions, this one is quite generic, and can be hard to implement—even with a streamlined plugin like WP Rocket!
WP Rocket has 2 options to address the PageSpeed recommendation:
By enabling these options you may see an improvement in your performance grade on GTmetrix, Pingdom Tools, and Google PageSpeed Insights. Results will vary from site to site. Always remember to test the effect on the speed of your site.