This new update is not really good. We facing lot of problems like apps are taking too long to open and difficulties in playing game and battery draining so fast and the the fb messenger cant have the normally chat heads which we used to have it before.....personally i hate this.
Samsung you have to fix this.
تم الحل! انتقل إلى الحل.
Try to wipe cache partition via recovery mode. Regarding battery consumption, you need to discharge your battery several times in order to judge whether the update drains your battery life or not
Although I don't have the OneUI 3.0 yet, but I believe the FB messenger's chat-heads feature is about permissions, you need to give it proper permission to display over apps, either from permissions manager or from app details