Changed display refresh to 120, device sluggish after that. It got quite warm. Some minutes later device unresponsive including power side button. Eventually a response and able to shut down the phone. After 5 mins, restarted successfully. So far no repeat of this issue.
But, my phone is now pretty sluggish. I've reverted to a display refresh of 60 otherwise the phone is not very usable at 120. Clearly not a desired outcome.