Not worth it.

The plugin does what it says. FYI I’ve found that it conflicts with the AMP plugin, and maybe it conflicts with some other plugins as well, but for the most part it does what it says.

But it also has a popup on every single admin pageview that prompts you to buy a pro version. Considering the extraordinarily simple tweak that this plugin provides, it’s not worth the annoyance.

Just add this at the bottom of your functions.php file instead:

/*function to add async to all scripts*/
function js_async_attr($tag){
    # Add async to all remaining scripts
    return str_replace( ' src', ' async="async" src', $tag );
}
add_filter( 'script_loader_tag', 'js_async_attr', 10 );

Source: https://orbitingweb.com/blog/add-defer-async-attributes-to-scripts-in-wordpress/