Last year I upgraded from a 6s+ to a 12 and I can confirm this the other way around: my old phone did feel slow for a couple of key apps; turns out they are still slow on the new phone. They are just poorly written, and one is basically just a CRUD app with no excuse.
So my lesson is to likely keep this phone for a decade. It's not like I couldn't afford to upgrade but why bother?
[happy owner of a 6s here]
Another big difference is 5G. I can easily get 500+ MBit/s downstream while outside.
Oh, and about the cameras: the one update I especially like is the night photography, I can now even take photos of the starry night sky.
Besides that: it’s just a phone, it just has an absurdly powerful CPU in it, and now it does text detection in images. Screenshots and photos have selectable text in it.
Yes, the battery was kinda shot but I could have replaced it.