Database Query of Users from UserlistProvider is very slow

Steps to reproduce:
1. Create several users > 200
2. create a new event
3. Open admin-ng/resources/USERS.JSON

Actual Results:
opening add event takes about 4sec
opening admin-ng/resources/USERS.JSON takes about 1sec
Loading time increases linear with the amount of users in the system

Expected Results:
Users-List should be available in under 100ms with about 800Users in the system.

Workaround (if any):