The models that can be upgraded this time include iPhone 5s, iPhone 6, iPhone 6 Plus, iPad Air, iPad mini 2, iPad mini 3 and the sixth generation iPod touch, covering three categories of iPhone, iPad and iPod Touch, basically including the models in the last five years.
Many iPhone users often say that after the upgrade of the old model, the phone has serious fever and is stuck. In fact, a large part of these are caused by excessive wear and tear of batteries. After the battery performance decays to a certain extent, more voltage is needed to support the normal operation of the mobile phone, and then the problem of heating is easy to occur. Moreover, fever will cause the processor to reduce the frequency, which will bring the feeling of being stuck.
The batteries on Apple's early models were relatively small, and the capacity of many iPhone was less than 2000 mAh. Now, battery life has become one of the biggest problems. If you want to improve the experience, you might as well change a battery, which may be much better.