Are Sessions faster than running a processor intensive function?

I am using the native Wordpress function wp_nav_menu() to create my site's navigation menus. This function really takes a long time to work, especially if the navigational menus is large like mine is. So my thought to get around this is as follows:

session_start();
if(isset($_SESSION['topTranslucent']))
    echo $_SESSION['topTranslucent'];
else {
    // ob necessary because wp_nav_menu() echos it's results
    ob_start();
    wp_nav_menu(array('menu'=>'Top Translucent','container'=>'','menu_id'=>'topMenu'));
    $_SESSION['topTranslucent'] = ob_get_contents();
    ob_end_flush();
}

My thinking here is that it will be much faster to print out the html stored in the session variable than to rerun the function on every page load. But not being too experienced with php sessions, I wanted to get some expert opions f开发者_高级运维rom you lovely wunderkinds at StackOverflow. Question is: Are sessions actually just doing what they seem to be doing? (i.e. storing text data in a cookie to be used across pages), or is there more than meets the eye?

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Sessions are storing the serialized data on the server; they use cookies to for identification only. Example:

Client:
  cookie { PHPSESSID => '1234567890a' }

Server:
  cookie { PHPSESSID => '1234567890a' }
  => session 1234567890a {
     topTranslucent => '<yourcode>whatever</yourcode>'
  }

Your approach could work; note that the whole session will be unserialized on load (so overusing this will slow down the system, as it will load a lot of data. Using this for a few small snippets should be OK).

Possibly a better approach would be using a different mechanism as a cache, but sessions-as-a-cache are somewhat usable.