Add lazy attributes to all img tags in WordPress.

lazy-load-images.php

<?php
/**
 * Add loading="lazy" and decodiing="async"
 *
 * @see https://spelldata.co.jp/blog/blog-2019-12-19.html
 */

/**
 * Start buffer for img attributes.
 */
add_action( 'wp_head', function() {
	ob_start();
}, 9999 );

/**
 * Replace img tag and output everything.
 */
add_action( 'wp_footer', function() {
	$body     = ob_get_contents();
	$replaced = preg_replace_callback( '#<img([^>]+)>#u', function( $matches ) {
		list( $match, $attr ) = $matches;
		foreach ( [
			'decoding' => 'async', // This line might be meaningless.
			'loading'  => 'lazy',
		] as $key => $val ) {
			if ( false !== strpos( $attr, $key . '=' ) ) {
				// If already added, skip.
				continue;
			}
			$attr = sprintf( ' %s="%s"%s', $key, $val, $attr );
		}
		return sprintf( '<img%s>', $attr );
	}, $body );
	ob_end_clean();
	echo $replaced;
}, 9999 );