[dml2135]

> getting too slow to be usable

Not sure what the behavior is like on Android, but iOS will throttle performance if your battery has degraded past a certain state. So I'm sure that there are many iPhone users that are replacing their phone due to what they think is poor performance related to the age of the phone, when it's really due to the age of the battery.

[boxed]

I think the iPhone will warn you if your battery performance is significantly degraded?

[dml2135]

It's buried in the battery settings, so not something you will find unless you are looking for it, IIRC.

[rootusrootus]

You can find it under settings, but when it goes below 80% the phone will start telling you about it regularly using a big prominent notification. Especially if it causes a crash.