Why is my 'list_terms_exclusions' filter function timing out? [closed]

Question

Why is my function timing out?

When the issue was first noticed I added wp_defer_term_counting('true') to delay that process but to no avail.

/* Filter category from get_categories and wp_list_categories */

add_filter('list_terms_exclusions', 'my_list_terms_exclusions', 10, 2);

function my_list_terms_exclusions($exclusions, $args) {

    global $my_sc_id;

    // Defer term counting as its a heavy operation and may time out the server
    wp_defer_term_counting('true');

    if (!is_admin() && $my_sc_id) {

        //Get the secret category and all its descendents
        $list_terms_exclusions = my_sc_return_secret_cats($elbone_sc_id); // Returns an array

        // Implode array
        $list_terms_exclusions_string = implode(',', $list_terms_exclusions); // Returns a string

        // Filter
        $list_terms_exclusions_clean = wp_parse_id_list($list_terms_exclusions_string);

        // Add to SQL query
        foreach ($list_terms_exclusions_clean as $ex) {

            if (empty($exclusions))
                $exclusions = ' AND ( t.term_id <> '.intval($ex).' ';
            else
                $exclusions .= ' AND t.term_id <> '.intval($ex).' ';

        }

        // Closing bracket
        if (!empty($exclusions))
            $exclusions .= ')';

        // Modify query
        return $exclusions;     

    }

    // Enable term counting
    wp_update_term_count();


}

EDIT: The contents of the elbone_sc_return_secret_cats function.

/* Return child categories as array */

function elbone_sc_return_secret_cats($elbone_sc_id) {

    // Add the child cats to the array  
    $sc_children = get_terms('category', 'child_of='.$elbone_sc_id);

    foreach ($sc_children as $sc_child) {

        $sc_child_ids[] = $sc_child->term_id;

    }

    // Add the parent category
    $sc_child_ids[] = $elbone_sc_id;

    // Return results
    return (array)$sc_child_ids;

}