Depending on a variety of conditions, you may see a message on your iPhone during live-streaming that your device needs to cool down, so we've reduced the quality of your stream. You can select OK, and continue your stream.
The reason this occurs is that your iPhone has built in features to protect your phone from apps overheating it. If you see this message, it means that the quality of each frame is the same, but the number of frames "sent" may be reduced. Similar frame drop occurs under bad network conditions, for example.