Hi Helpfo,
We understand that you're experiencing some problems with your iPhone XR after installing an update, and we'd like to help. When you say the app is freezing, do you mean that it doesn't open, or that the app stops responding once it's open? If the app is running slowly while it's open, is the iPhone connected to a Wi-Fi network, or to cellular data? The reason we ask is that the network speed can affect how quickly the app responds.
Also, if you open Settings > Battery > Battery Health, what do you see next to Maximum Capacity? This number should be between 80-100%, and will tell us if the battery is still able to support iOS 15 or if it should be replaced. The article below has some more details as well.
About iOS performance on your iPhone, iPad, or iPod touch
We look forward to learning more, and continuing to assist!
Have a great day.