Image Slider and Render-blocking JavaScript
Google recommends to load the JavaScripts after html, css and images.
There are two ways to fix the Eliminate render-blocking JavaScript issue.
1. To add Defer attribute (I do NOT recommend)
2. To move the JS sources to the bottom of the page
WP Fastest Cache Premium uses the second solution.
Image Slider
As I said above that the JS sources are loaded at the bottom of the page if you use the premium version of WP Fastest Cache.
If you use a image slider, the slider loads the images after page loaded.
In the conclusion, if you use a image slider, you cannot fix the Render-blocking JavaScript warning.