Phone got a little warm (definitely not hot or uncomfortable to hold) while syncing iCloud Photos, It actually paused for a few minutes due to heat. Been fine since then.
I’m not too worried overall, but this is an area where it seems like Apple can improve. Top notch chip, top notch materials/display, it would be nice to see them improve the cooling architecture. I wonder what the trade offs are (other than cost, of course).
If the only trade off was cost I think Apple would have improved thermal management years ago.
Perhaps to make a real dent in the heat it would take up too much space and Apple has determined the trade off isn’t worth it yet.