Not sure how much degraded battery affects it. I recently got my iPhone 8 degraded because of battery and honestly I didn't notice anything.
Even my old iPhone 4S is a little beast. I remember playing Asphalt racing game. It pushed it to 60 FPS and its graphics was stunning, better than need for speed. If phone can render AAA game at 60 FPS but struggles with some chat app, this chat app does not respect is users. I mean it's just a bunch of grey squares with some text and lame shadows. DOS on i386 could render it fast enough.
Sometimes I can't decide who's more crazy - me who demands adequate performance from simplistic apps or people who accept this state of things.
Ehhh that's a >5 year old CPU. I wouldn't describe it as "extremely powerful" even on a phone.
Anecdotally, going from an iPhone 8 to a 13 pro max was a night and day difference. Not just because the CPU was noticably faster, but because the 120Hz display makes everything more responsive. Sure you don't need that hardware for a chat app but unless some product guy forces the issue, devs aren't even gonna notice fps dropping below 120 because most of them don't use ancient phones.
I always wonder how non-HNers use such software, if even the dedicated people are struggling.