How to mark your WordPress scripts as async or defer. For more info see https://philkurth.com.au/tips/how-to-set-defer-or-async-attributes-on-enqueued-script-tags-in-wordpress/

mark-wordpress-scripts-as-async-or-defer.php

<?php

add_action( 'wp_enqueue_scripts', function () {

	wp_register_script( 'my-script', get_stylesheet_directory_uri() . '/assets/js/my-script.js' );
	wp_enqueue_script( 'my-script' );

} );

add_filter( 'script_loader_tag', function ( $tag, $handle ) {

	if ( 'my-script' !== $handle ) {
		return $tag;
	}

	return str_replace( ' src', ' defer src', $tag ); // defer the script
	//return str_replace( ' src', ' async src', $tag ); // OR async the script
	//return str_replace( ' src', ' async defer src', $tag ); // OR do both!

}, 10, 2 );