transients

in functions.php

function list_all_agent_posts() {
  // Configuration
    $transName = 'tr-agent-posts'; // Name of value in database.
    $cacheTime = 720; // Time in minutes between updates.
  // Do we already have a saved transient?
    $agent_posts = get_transient($transName);
  // If not, lets get one.
    if($agent_posts === false):
      $agent_posts = new WP_Query(array('post_type' => 'agent', 'order' => 'ASC', 'orderby' => 'title'));
  // Save our new transient.
    set_transient($transName, $agent_posts, 60 * $cacheTime);
    endif;
  // send back the result.
    return $agent_posts;
}