Preload First Post Image

function preload_first_content_image() {

    if (is_single()) {

        $post = get_post();

        if ($post) {

            // Get post content

            $content = $post->post_content;

            // Find first image in the content

            preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $content, $matches);

            if (!empty($matches[1])) {

                $first_image = $matches[1][0];

                echo '<link rel="preload" as="image" fetchpriority="high" href="' . esc_url($first_image) . '">';

            }

        }

    }

}

add_action('wp_head', 'preload_first_content_image', 1);