CREATE USER command slows down when user count per server reaches up to 500 000

When users 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.