author-filter-meta-box.php

<?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 Bueltge
 * 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', __('Authors Only'), '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_Query(array(
        'query_id' => 'wps_not_subscriber',
        'fields' => 'ID',
        'meta_query' => array(
            array(
                'key'     => 'wp_capabilities',
                'value'   => 'subscriber',
                'compare' => 'not like'
                )
            )
        )
    );
    $authors = join( ', ', $wp_user_search->get_results() );
    ?>
    <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
    ) );
}