• I've read and understood the contribution guidelines.
• I've searched for any related issues and avoided creating a duplicate issue.

Please give us a description of what happened

To Reproduce

Step-by-step reproduction instructions

1. Have a lot of user in your database with certain rights.
2. Go to a post edit page where it loads the Comment notification recipients dropdown.
3. Page loads stop because of high memory usage.

Expected results

1. Page to load.

Actual results

1. Page doesn't load.

In the admin.php on you use wp_dropdown_users to load the users. It would be nice if this could be replaced by an AJAX-powered search field so it doesn't load all the users at the same time.