When the user starts collecting data using devices running on 2GB RAM, the app slows down where you can type a text and shows some delays.
The app can allow you to log in when there is internet for the first time. But when you try it when there is no internet connection, the app does not allow that.
Run the application on devices running on 3GB RAM, start contact without closing the application, and do it for at least three clients. When you reach on tests tab, when you type some text on EditText you will observe performance issues by typing a text.
(This might be caused by validation)