Inconsistent animation smoothness on iPhone 15

There may be several reasons why a device may behave as described by you. Some of them could be...

Let's delve a bit deeper into some of them:

1. iPhone Storage: A full iPhone can slow things down. Phones can experience performance issues when the storage is nearly full. Users can check their storage status by going to Settings > General > [Device] Storage. Deleting unnecessary apps, photos, or videos can free up space and improve performance. How to check the storage on your iPhone and iPad - Apple Support.
2. Device's Battery Health: A degraded battery can impact performance. A deteriorating battery can impact the overall performance of an iPhone. Checking the battery health can be done by going to Settings > Battery > Battery Health. If the maximum capacity is significantly reduced, it might be time to replace the battery. Prolong and Maximise Battery Performance … - Apple Community
3. Incorrect Handling of the device: Accidental drops or bumps can cause internal damage. Looking at Apple's guide on proper handling is a good step. Important handling information for iPhone - Apple Support (IN)
4. Environmental Issues: Extreme temperatures, dust, and water exposure can all be detrimental.
5. Keeping the iPhone/Watch and Airpod clean is an important step. Dust accumulation in the device's ports and openings can affect functionality. Apple provides guidelines on how to clean iPhones safely, which usually involves using a soft, lint-free cloth to remove dust without causing damage.
   1. 1. Cleaning your iPhone - Apple Support
      2. How to clean your Apple Watch - Apple Support (IN)
6. Clean your AirPods Max - Apple Support
7. Keep the iPhone within temperature limits. Phones are designed to operate within a specific temperature range. Exposure to extreme temperatures, especially in parked cars, can lead to overheating or other performance issues. If your iPhone or iPad gets too hot or too cold - Apple Support.
8. Addressing safety issues is also important: Important safety information for iPhone - Apple Support (IN)
9. Unauthorized Service: Servicing a device through unauthorized channels can result in substandard repairs and void warranties. Users should always seek authorized Apple service centers for repairs and maintenance.
   1. Repairs by unqualified technicians can introduce new problems. Find information about your warranty or AppleCare plan - Apple Support is helpful.
   2. Modification / Alteration of a device by a third-party vendor may cause issues beyond repair. --> Unauthorized modification of iOS - Apple Support (IN)
10. Jailbreak on iPhone: Now, let's talk about jailbreaking. Jailbreaking removes Apple's restrictions, allowing for more customization and app installation outside the App Store. However, it can also lead to instability, security vulnerabilities, and issues receiving future iOS updates.

By considering all these possibilities, you've provided a comprehensive approach to troubleshooting the iPhone's behavior.