iPhone X throttling

I have a 18 month old X and haven’t noticed this type of behaviour, you have checked the battery so that was my first thought, second would be temperature, does the device get excessively hot at the 15 minute period and are you using a heavily insulated case.

You could try a Forced Restart to see if that clears up or resets anything, you could also do a Restore as New via iTunes and test, if ok then re-apply your BackUp and then see what happens.

A Restore not only wipes out Settings and support files but also replaces the OS, so if anything strange has happened with previous updates it may sort, but using the BackUp may bring the issue back again, it’s something you will have to decide on when faced with the decision.

It sounds rather like over-temperature protection might be activating due to continuous intensive processor usage, but with only an internet description there is no way we can possibly guess the underlying reason. Contact Support and arrange to get your iPhone checked by Apple.

If you are using a case on your iPhone test without the case to see if it improves heat dissipation. This will also allow you to feel the temperature of the phone’s body which might give you an indication whether heat is involved.

Not sure if this will help but have you checked which apps appear at the top under Battery Usage, particularly the ones you are not actively using like Mail or Social Media types.

It could be some of them are doing a lot of activity in the background like fetching for Mail which if there are a few accounts can all add up, and when you decide to play an intensive game it pushes the device to the limit.

Just a thought.

Apple recommend two options, either Update/Restore normally or putting the device into Recovery Mode, there is a third namely DFU Mode which is the only one I believe that’s capable of replacing the firmware.

I always tend to update via iTunes now following issues in the past with OTA, but that’s just my preference, there’s something reassuring watching the OS getting replaced rather than just being added too.