wp_dropdown_users running out of memory

Running a client site with >200,000 users. They would like to be able to delete users but cannot currently as, whenever they hit ‘Delete’ on a user and are taken to the ‘Delete Users’ screen, the user dropdown does not appear and neither does the ‘Confirm Deletion’ button. The page is running out of memory.

Am using add_filter('wp_dropdown_users', 'custom_function') to try and override the function, but no matter what I do here, the page is still running out of memory.

Of course I don’t want to change any core files, but does anyone know a way to fix this? This appears to be a pretty serious problem… maybe one that WordPress needs to address…?