How to Optimize the Divi Gallery Module | http://www.ibenic.com/optimize-divi-gallery-module

regex.php

<?php

/**
 * Replacing the original image source with a placeholder
 * Placing the original source under attribute data-original    
 * @param  array $img_match 
 * @return string            
 */
function my_preg_lazyload($img_match) {
    
    $img_replace = $img_match[1] . 'src="https://via.placeholder.com/380x250?text=Image%20Loading" data-original' . substr($img_match[2], 3) . $img_match[3];
 
    $img_replace = preg_replace('/class\s*=\s*"/i', 'class="lazy ', $img_replace);
 
    $img_replace .= '<noscript>' . $img_match[0] . '</noscript>';

    return $img_replace;
   
}

script.js

(function($){

	$(document).ready(function(){
     
     		// Show the first image of each gallery
		 $(".et_pb_gallery_items").each( function(){

		 	var first_image = $(this).find('.et_pb_gallery_item').eq(0).find('img');
		 	var orig = first_image.attr('data-original');
			first_image.attr("src", orig);
		 });

	});

})(jQuery);

script2.js

(function($){

	$(document).ready(function(){
		// ...
		
		// Trigger this before the next gallery image loads 
		$(document).on( 'simple_slider_before_move_to', function( $arg, $arg2 ){
			var $original_img = $arg2.next_slide.find('[data-original]');
			var $orig = $original_img.attr('data-original');
			$original_img.attr("src", $orig);
		});

	});

})(jQuery);

shortcode.php

<?php

add_filter( 'do_shortcode_tag', 'my_gallery_shortcode', 99, 4 );

/**
 * Replacing images SRC in Divi Gallery 
 * @param  string $output 
 * @param  string $tag    
 * @param  array $attr   
 * @param  array $m      
 * @return string         
 */
function my_gallery_shortcode(  $output, $tag, $attr, $m ) {
     
    if( 'et_pb_gallery' == $tag ) {
        
        // This script can be used just for these if you don't have another JavaScript file
        if( ! wp_script_is( 'my-gallery-enh' ) {
            wp_enqueue_script( 'my-gallery-enh', get_stylesheet_directory_uri() . '/assets/public/gallery.js', array( 'jquery' ), '1.0', true );
        }
        
        return preg_replace_callback('/(<\s*img[^>]+)(src\s*=\s*"[^"]+")([^>]+>)/i', 'my_preg_lazyload', $output );
     }

    return $output;
}