Limit user list in admin when deleting users

Hi all – I manage a WP site with over 270k subscribers. The large volume of subscribers has no noticeable effect on performance, however one issue I can’t figure out how to resolve is this:

When we have to delete a user from time to time, you have to wait for the entire list of subscribers to load in the drop down menu for “Attribute all posts to” on this page in the admin:

/wp-admin/users.php?action=delete&user=[user id]&_wpnonce=[blahblahblah]

Sometimes the page will actually finish loading, sometimes it won’t (thus the confirm deletion button remains disabled).

Is there a filter I can run on just this page so that the drop-down only includes contributors and above? We’re not using any special user levels or anything like that. Really would like to avoid having to hack a core page.

Thanks very much!