On a similar note, I have a Motorola G8+, Android 10. It has 4GB of RAM, plenty of free space in storage, and the CPU isn't actually that bad still. The battery is in good condition. A brand new phone doesn't necessarily have any better stats.

For some reason, it is slow. As in, it will still usually run apps at full speed, the framerate in a flight sim is just as high as before etc, but then it will just freeze up for seconds at a time, or suddenly do something really slowly for a while. Sometimes it will fail to respond to the power button short-press to turn the screen on, but if I try again a minute later it will be fine.

I have experienced this with multiple Androids over time. My previous phone got so bad that it sometimes wouldn't actually respond quickly enough to be able to answer a phone call, which is when I got the new one.

What causes this? Is there some build-up of junk lying around in the system that I could press a magic button and clear out, without having to reinstall all my stuff from scratch?

I could buy a new phone. But given a new one wouldn't have much more resources than my current one, it very much feels like the wrong workaround for a problem that shouldn't exist.