They are for those tasks, where you do need high performance. Where you would wait for your device instead. A few tasks require all cpu power you can get, so that is what the performance cores are for. But most of the time, it will consume a fraction of that power.
My whole point is that iPadOS is such that there's really nothing useful to do with that performance. No task an iPad can do requires CPU power (except for maybe playing games but that'll throttle the M4 to hell and back anyway).
So what are the performance cores doing there?