I have been using Google Duo to tutor and sometimes the audio becomes very delayed while the video is working fine. I would be able to see the person talk but then the audio from what he said comes 3-10 seconds later. Both of our internet connections and battery charge were fine when this would occur. After the problem would get worse, I would hang up then call back. The delay would reset and then get worse after a while again. I am using an android phone and the other caller is using iOS.