How to speed up you your Divi gallery if you are using only the first thumbnail. See the related post here: https://www.stefanocappellini.it/posts/posts/make_divi_galleries_run_faster_without_thumbnails/

functions.php

<?php
// Add this to your Divi child theme' functions.php file
// Remember to add the my_divi_fast_gallery class to all the galleries you want to optimize
add_filter('do_shortcode_tag', 'fast_gallery_tip', 105, 2);
function fast_gallery_tip($output, $tag){
    if('et_pb_gallery' === $tag && strpos($output, 'my_divi_fast_gallery') !== false){
        $pattern = '/<img[^>]*>/';
        $index = 0;
        
        return preg_replace_callback($pattern, function($matches) use (&$index) {
            return $index++ === 0 ? $matches[0] : sprintf('<img src="%s/square.jpg"/>', get_stylesheet_directory_uri()); 
        }, $output);
    }
    return $output;
}