Improve notification for larger users

[rizaardiyanto1412]

So we have 135k users in our database, and we found out one of the reasons our posts are so slow is because on every page, for the “Enter any users, roles, or email address that should receive notifications from workflows” menu, it loads all 135k users into one menu.

Is there any possibility of slimming that down or making it into an AJAX-based autocomplete?

Original request: https://wordpress.org/support/topic/not-listing-all-users-in-the-workflow-notification-on-posts/