Cache a slow WP_Query in WordPress

functions.php

<?php
/**
 * Function that gets  recent posts
 */
function get_recent_posts() {

	//No cache
	if(!wp_cache_get('my_complex_query_result')) {

		//This is the super slow query.
		$query = new WP_Query(array(
			'posts_per_page' => 3
		));

		//Grab the ID:s so we can make a much more lightweight query later
		$post_ids = wp_list_pluck( $query->posts, 'ID' );

		//Cache it!
		wp_cache_set('my_complex_query_result', $post_ids);

		$posts = $query->posts;
	}
	//Cache!
	else {

		//Lightweight query using post__in
		$query = new WP_Query(array(
			'post__in' => wp_cache_get('my_complex_query_result'),
			'posts_per_page' => -1
		));

		$posts = $query->posts;
	}

	return $posts;
}

/**
 * Invalidation function. When posts get saved or published we delete the saved post list
 * because it may have changed.
 */
add_action('save_post', function() {
	wp_cache_delete('my_complex_query_result');
});

/**
 * Run this function anywhere. First run will do the heavy WP_Query and
 * the others will do a fast one
 */
get_recent_posts(); // 8 query overhead
get_recent_posts(); //Only 1 query overhead
get_recent_posts(); //Only 1 query overhead