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

    // Your custom class dir
    define('CLASS_DIR', 'class/')

    // Add your class dir to include path
    set_include_path(get_include_path().PATH_SEPARATOR.CLASS_DIR);

    // You can use this trick to make autoloader look for commonly used "My.class.php" type filenames
    spl_autoload_extensions('.class.php');

    // Use default autoload implementation
    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!