Apple since the battery health is less than 80%, will reduce the frequency, of course, health is not the only factor, Apple will be based on the device temperature, battery power, battery impedance, comprehensive these three variables, dynamically adjust the highest performance of CPU, GPU, etc.
The reason for the decline in battery capacity is that the battery is a consumable, and when the battery is working, the internal chemical reaction of charging or discharging has been carried out, and it will inevitably be consumed during the reaction process.
During the use of the battery, there will be lithium ions that are "locked", so that the power will slowly become smaller. This is also the reason why the performance of the battery decreases as the number of charges and discharges increases.
When should the battery be replaced? According to Apple's official statement, the normal iphone battery design, after 500 to 800 charging cycles, the battery health may be about 80%, as long as it is less than 80%, the battery life is reduced, as the battery internal resistance increases, it may also lead to "frequency reduction", the running speed is slow, at this time the battery should be replaced.
Original battery or third-party battery? Under normal circumstances, the replacement of the original battery is more "reliable", some high-capacity batteries, although the capacity is higher, but "not durable", after several charging and discharging cycles, the loss is serious, and the battery performance is reduced faster
However, the original battery has two shortcomings: first, the price is expensive, compared with the original battery, the third-party battery is less than 150, the price is cheap; Second: the replacement procedure is troublesome, the replacement of the original battery needs to be comprehensively tested, the waiting time is long, and the third-party battery is bought and replaced without waiting.
If it is an older model, then it may be more cost-effective to replace the third-party battery, and if it is a new model, I think it is more advisable to replace the original battery.