BUG #4919: CREATE USER command slows down system performance

The following bug has been logged online:

Bug reference:      4919
Logged by:          Lauris Ulmanis
Email address:      lauris.ulmanis@mykoob.com
PostgreSQL version: 8.3.7, 8.4.0
Operating system:   Any
Description:        CREATE USER command slows down system performance
Details:

When user count in Postgres database reaches up to 500 000 - database
command of creating users 'CREATE USER' slows down to 5-10 seconds per user.


What could be a reason of this problem and is there any solution how to
avoid it?

For each of user can be associated up to 10 roles with grants to system
objects.