This meta box is using the function wp_dropdown_users() to list all users. It has only one filter to change the output: wp_dropdown_users.
But this filter offers just the complete markup and users. It’s neither easy nor fast to parse this via regex for user names, check their roles and then give back an new HTML.
I think the best way is to deactivate this meta box.
remove_meta_box('authordiv', 'post', 'normal');
And then you create a custom meta box via plugin and list the authors of your blog only.
This small plugin does exactly that.
<?php
/**
* Plugin Name: Author Meta Box only with authors
* Plugin URI: http://wordpress.stackexchange.com/questions/60429/stop-loading-collaborators-users-on-add-new-post-or-page
* Description:
* Author: Frank Bültge
* Author URI: http://bueltge.de
* License: GPLv3
*/
add_action( 'admin_menu', 'fb_remove_author_meta_boxes' );
function fb_remove_author_meta_boxes() {
remove_meta_box('authordiv', 'post', 'normal');
add_meta_box('fb_authordiv', __('Author'), 'fb_post_author_meta_box', 'post', 'normal', 'core');
}
function fb_post_author_meta_box( $post ) {
global $user_ID;
// get all authors
$wp_user_search = new WP_User_Search( '', '', 'author' );
$authors = join( ', ', $wp_user_search->get_results() ); // user IDs
?>
<label class="screen-reader-text" for="post_author_override"><?php _e('Author'); ?></label>
<?php
wp_dropdown_users( array(
'include' => $authors,
'who' => 'authors',
'name' => 'post_author_override',
'selected' => empty($post->ID) ? $user_ID : $post->post_author,
'include_selected' => true
) );
}