Nextcloud Talk with LDAP crashes server


we started using Nextcloud Talk (desktop and mobile app) in our school. Writing messages is fine.
Our users are imported via LDAP from Microsoft AD (>1300 users, several groups).

First problem is, that it takes a lot of time to start a conversation on the smartphone, because you have to wait until all users are loaded before you can select the users you want to talk to.

Second problem is, that you can select a group with lots of users (>1200) and for example start a video call to all. That was the first idea our pupils had and in crashed the server completely.

Is there any possibility to deactivate speech and video calls?
Is it possible to hide groups when you select users to talk to?


Nextcloud 15.08
Ubuntu 18.04