Note, that the default autoload implementation is written in C land and is always slightly faster then your native PHP one.
Here is a trick to use default implementation with any configuration:
<?php
define('CLASS_DIR', 'class/')
set_include_path(get_include_path().PATH_SEPARATOR.CLASS_DIR);
spl_autoload_extensions('.class.php');
spl_autoload_register();
?>
This also works with namespaces out of the box. So you can write code like "use My\Name\Object" and it will map to "class/My/Name/Object.class.php" file path!