PHP register_shutdown_function 1 second delay

I'm trying to figure out why PHP takes 1 full second to execute a register_shutdown_function. And if there is a way to overcome this.

<?php

$start_time = microtime(true);

function shutdown($start_time)
{
    $time_diff = round(1000 * (microtime(true)-$start_time));
    echo $time_diff . "ms<br/>";
}

register_shutdown_function('shutdown', $start_time);
register_shutdown_function('shutdown', $start_time);
register_shutdown_function('shutdown', $start_time);
register_shutdown_function('shutdown', $start_time);
register_shutdown_function('shutdown', $start_time);

Outputs:

1001ms
2001ms
3003ms
4003ms
5003ms

Running XAMPP 1.6.8 on Windows / PHP: 5.2.6