Improve User creation time for large-scale instances

Issue Summary

The time required to create a new user in Bitbucket's internal directory doesn't increase linear, but exponential. For instances with more than 10 000 users, it can take more than 10 minutes to create a single user.

Steps to Reproduce

1. Create 15-20k users making sure that parameter addToDefaultGroup==true
2. Measure time elapsed on each user's creation.

Expected Results

The time required to create a new user should be considerably lower and adding new users shouldn't cause the time to increase exponentially, but linear.

Actual Results

In vanilla Bitbucket (no projects, no repos, 5 groups, separate AWS instance, no traffic), the time it takes to create a new user grows exponentially:

Workaround

Add external User Directory and start adding users to that directory, instead of creating users directly in Bitbucket. It can be any type of external directory, including Delegated LDAP authentication.
Please be advised once you exceed your license a banner in licensing will be displayed:

You have more users than your license allows. Pushing will be disabled in 23 hours.
Users will not be able to push commits to repositories until you restrict the number of users to match your license or upgrade your license.