Recently there has been performance issues in the iOS app when learning or reviewing. Sometimes it is not possible to enter an answer for a few seconds after answering the previous one correctly, it only becomes possible to continue after the small firework animation in the top right corner has been shown. So the question is shown directly but nothing happens it you click on any buttons.
It happens frequently for me but not in every session. It’s been the case with the recent app builds on iPad with the latest iOS.