There’s an interesting thread on Treklite forum about how the performance of iPhones degrade in cold weather https://www.trek-lite.com/index.php?threads/iphones-in-cold-weather.7417/page-2 . To be fair it’s not just iPhones, pretty much any mobile phone or camera will suffer. Batteries tend to drain and phones can even shut down if it gets too cold.
I have an iPhone 6 and over the past couple of years I’ve found that the battery drains more quickly and erratically even after having a new battery fitted. Part of the problem is an old processor running newer operating systems and apps, having to work harder. Putting the phone into airplane mode and using the battery save setting helps.
A couple of times, I’ve found the battery drain quite a lot overnight, which is probably down to lower temperatures. Having an insulated pouch in which to put the phone overnight I found very helpful in maintaining battery charge on my recent trip. As I mentioned in a previous post, I used a Valley & Peak pouch which doubles as a cosy for freeze dried meals. It’s well worth considering to protect electronics and for food rehydration.