cache-post-thumbnails.php

<?php
/**
 * Plugin Name: Cache Post Thumbnails
 * Description: Prime the post thumbnails cache for individual loops.
 * Version: 1.0.0
 * Author: Brady Vercher
 * Author URI: http://www.blazersix.com/
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

/**
 * Prime post thumbnail cache.
 *
 * Automatically primes the cache for the main loop on the front page and
 * archives. The cache can be activated for additional queries using the
 * 'blazersix_cache_post_thumbnails' filter.
 *
 * Custom queries can also pass a 'blazersix_cache_post_thumbnails' flag via the
 * query args to prime the cache.
 *
 * @since 1.0.0
 *
 * @param array $posts List of posts in the query.
 * @param WP_Query $wp_query WP Query object.
 * @return array
 */
function blazersix_prime_post_thumbnails_cache( $posts, $wp_query ) {
	// Prime the cache for the main front page and archive loops by default.
	$is_main_archive_loop = $wp_query->is_main_query() && ! is_singular();
	$do_prime_cache = apply_filters( 'blazersix_cache_post_thumbnails', $is_main_archive_loop );

	if ( ! $do_prime_cache && ! $wp_query->get( 'blazersix_cache_post_thumbnails' ) ) {
		return $posts;
	}

	update_post_thumbnail_cache( $wp_query );

	return $posts;
}
add_action( 'the_posts', 'blazersix_prime_post_thumbnails_cache', 10, 2 );