system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does some smartphones get slower with time? Is it a hardware or a software problem?
gpt
Both

On the software side of things not only are newer programs typically more demanding, but newer operating systems are designed with newer/more powerful devices in mind. 

As for hardware, the only real thing that degrades over time is the battery. As they age, and go through more charging cycles, batteries tend to be able to store less charge. What's more important for performance though, is that they also can provide less current. As electronics are under more load they draw more current, and if the aging battery isn't able to keep up then that effectively puts a speed limit on the device as a whole.