How do I check if a user is logged in?

I am writing a small JavaScript/PHP application that is not a WP plugin. The application stores options tied to the WP user id (because it is connected to my blog and I want to work with the existing accounts).

It checks if a WP user is logged in by calling wp_validate_auth_cookie(). This works BUT it also generates a lot of SQL on each call: 7 selects on wp_options, 1 delete on wp_options, 1 gigantic insert on wp_options. I uninstalled all plugins and restarted Apache but still see the problem.

This is the PHP I’m testing with

<?php
// vvv CHANGE NEXT LINE TO POINT TO YOUR wp-config FILE vvv
require_once getenv("DOCUMENT_ROOT")."/wp-config.php";
$userId = wp_validate_auth_cookie();
echo "USER: ".($userId?$userId:"no user");
?>

Any ideas what could be going on?