Battery can degrade from a number of reasons, but the main 2 reasons are extreme temperature (especially hot temperatures) and keeping the battery fully charged for a long time without discharging. But a brand new battery shouldn’t drop 4 - 5% after a software update.
This has happened to me before. After updating the software, battery health dropped a few percentages quickly. Not sure if Apple is trying to mess with us or not.
Otherwise, do a clean factory reset first and wait a few weeks until the operating system can optimize. If it still freezes and hangs, then it’s likely caused by the iOS 17 software, might need to wait until it gets fixed in the next update.
That looks very normal. The 2016 iPhone SE does not have good battery life, even if your battery is in good condition, that’s just how it is unfortunately. It is using a 14nm CPU which is outdated and not very power efficient, plus the battery capacity is 1624 mAh. I notice the battery life is worse when there’s heavy WiFi activity (such as downloading apps via WiFi). Streaming audio and video isn’t too bad in comparison (especially if you stream at lower quality).
It’s also possible that the iPhone software has not been fully optimized, you may need to wait a few weeks before the iOS system will optimize itself (it’ll know what apps/services you use the most and prioritize those apps while putting the rest to sleep). This process can take a few days or weeks.
Also the battery software might not be calibrate. It needs to go through at least 3 full charging cycles (100% to 0% 3 times). There are iPhone battery recalibration instructions which you can look up, but the quick instructions:
Apple’s battery software is sometimes not very good (the battery percentage goes up or down when you restart or turn on/off the iPhone). So you may need to recalibrate the battery. This issue is more likely to happen after a battery was recently replaced, the software was updated, or if the iPhone hasn’t been used in a long time.
The problem with buying used phones, it’s the same as buying a used car… You’ll never know what was done to the phone or what may have been repaired. It might be possible that the battery was replaced with a generic aftermarket battery which shows the battery health at 100% but the actual battery life isn’t really good. You can ask the seller for details on the phone and if they used an aftermarket battery or genuine battery. Since it’s a 2016 iPhone model, it’s very likely the battery was replaced as it’s an old phone and there’s no way it could still have 100% battery health unless the battery was replaced.
A lot of people, especially Apple fans and so-called mainstream tech reviewers, often do not criticize Apple or point out its flaws. The iPhone is notorious for having poor thermal cooling capabilities, the CPU often gets hot according to thermal tests. Apple doesn’t want to put a copper heatsink or other thermal cooling hardware inside their iPhones. The hot CPU can cause the battery health to degrade sooner. Some Android phones have very good cooling systems, usually a copper heat pipe, a copper vapor chamber, or thermal pads. Another problem is Apple pushing their MagSafe wireless charger, but wireless charging is known to create extreme amounts of heat which significantly lowers the battery health within a short period of time. And also, Apple was so stubborn, they kept using the Apple lightning connector all the way to the iPhone 14 until the EU forced them to switch to USB C on the iPhone 15 and beyond. The Lightning connector is not very efficient and gets hot, especially when fast charging. So there are other ways the iPhone can get exposed to extreme heat which is not good for the battery health in the long run.
I suspect that Apple may purposely decrease the battery health number after a software update, probably to make people think that their battery health is lower than it actually is. This is because I also saw my battery health drop from 95% to 88% pretty quickly after a software update which was strange.