Very sure to tell you that the battery is down, change a battery is good
From your description it seems that your battery should be out of order, resulting in it showing that there is still 20% of the power, in fact, the power supply has been unstable, the mobile phone in order to protect itself, will become very very slow, you can connect your mobile phone to the computer, and then install an Aixis assistant, look at the battery wear situation, if it is below 70%, You should consider replacing the battery
Often use non-original data cable to charge, a long time of empty power will lead to a lack of battery power, 20% is only virtual power, it is recommended to use the regular original data cable to charge, as far as possible in the battery there is more than 30% of the power when charging will extend the battery life
The battery is not good to charge will be virtual, my previous also, sometimes put a long time to boot actually have more than 20 electricity, but it is also virtual, can only maintain the boot a use will be stuck or shut down
One of the above situations occurs, please replace a mobile phone battery in time to avoid affecting normal use
Cause: The free space of the iPhone is insufficient. At this time, you need to buy a new machine or expand the memory.
Cause: I downloaded an incomplete beta installation package over Wifi. It is also possible that this situation will cause the phone to stall.
Reason: iCloud is full of free 5G space