All rechargeable batteries wear out over time as they are used and recharged. And when batteries wear out, they can't provide the same voltage as before. And when they can't provide the same voltage, the electronics must either slow down or stop working entirely. Replacing batteries isn't necessarily hard to do either (you do have to be careful). You can find highly detailed descriptions of how to do it yourself if there isn't anyone in your area who can provide the service cheaply.

Battery is still good though, I don't use it much but after days it still has enough power. In terms of battery iOS is very good.