The app pushes the phone camera to maximum exposure and sensitivity to collect more photons. Because of that, the image may update slowly. That is not lag; it is the physics of night vision on a phone. Image quality depends on your phone model and on how much ambient light is available. Use it outdoors at night, where there is at least some light to amplify. Do not use it in daylight, and do not expect visibility in total darkness with zero light. In those conditions the screen can look white or the scene can stay black.
