Change the frequency of the Matador Scheduled Event / WP Cron task.

custom_matador_scheduled_event_recurrence__all.php

/**
 * Customize Matador: Set Recurrence of Scheduled Events
 * 
 * This function sets the recurrence of the Matador WP Cron from 'hourly'
 * to the returned value. Warning: returned value must exist in the WordPress
 * schedules array. Default values in the WP Schedules Array are 'hourly', 
 * 'twicedaily', and 'daily'. Example function uses a non-standard schedule,
 * which will need to be registered in a separate function. Also, Matador Software
 * provides this method as-is and will not support sites that run schedules at fewer
 * than once per hour. Running more regularly will risk exceeding daily rate limits
 * or causing a web host to suspend an account for too much CPU use.
 * 
 * @return string
 */
function custom_matador_scheduled_event_recurrence__all() {
	return '15min';
}
add_filter( 'matador_scheduled_event_recurrence__all', 'custom_matador_scheduled_event_recurrence__all' );

/**
 * Add Schedule to WordPress Schedules Array
 *
 * This function adds a custom schedule to the WordPress schedules array. It
 * is necessary to be included if you are trying to schedule Matador to run
 * at a different frequency than the WP default of 'hourly', 'twicedaily',
 * and 'daily'. If you are using a custom frequency in the function above,
 * the array key MUST match the string you are returning in the Matador call.
 *
 * @param array $schedules
 * @return array
 */
function custom_schedules( $schedules ) {
	if ( ! isset( $schedules['15min'] ) ) {
        $schedules['15min'] = array(
            'interval' => 15 * 60,
            'display' => __( 'Once every 15 minutes' ) );
    }
    return $schedules;
}
add_filter( 'cron_schedule', 'custom_schedules' );