The obvious answer is games. The kids play minecraft, which is probably the most demanding thing played on my phone.

The next is the camera. Phones often do some level of post processing, which the phone can struggle with. I've had phones in the past that you can only take one photo every 2 seconds or so, which seems plenty but then you miss the one good time when all the kids were looking at the camera at the same time.

And depending on what level of phone you go for, lower end phones feel sluggish, often very sluggish. Higher end phones just feel nicer to use, smoother scrolling, quicker when you tap things, etc. However, that doesn't mean they aren't perfectly usable, just that they feel sluggish in comparison.

Battery life can also vary wildly, but if you aren't using location tracking automation, watching Netflix, or taking lots of photos then the battery of most phones probably lasts plenty.