WordPress run its sheduled tasks using wp-cron.php, this get called every time visitor visits web site. This is not efficient method for high traffic web sites. Since this have to be loaded using web server, it can cause web server process hang if your cronjob take more time to finish. It is better disable wordpress cronjob and set a real cronjob.
To disable wordpress cronjob, edit wp-config.php file, add following
define('DISABLE_WP_CRON', true);
Now set a cronjob as follows
*/15 * * * * cd /home/user/public_html; php wp-cron.php &>/dev/null