I thought I'd share my experience here as well, being a long-time Lr on iOS user.
iPad:
I have 3 iPads: (1) a 2018 iPad Pro 11" 3rd gen A12X, (2) a 2023 iPad Pro 12.9" 6th gen M2, and (3) a 2024 iPad Pro 11" 7th gen M4. On all three iPads, if I am editing and scrolling through a lot of photos, the iPad gets hot in the area (presumably) where the GPU is. Sometimes it gets quite hot. It never gets hot enough to cause the screen to dim on any of my iPads, and the heat fades quickly when I stop flipping through photos or doing heavy edits. For example, last night after our local fireworks display on the 4th of July, I imported more than 250 full size CR3 raw files onto my iPad 7th gen M4, scrolled through them marking my picks, went back and edited the picks and exported some back to the camera roll. A little heat during the import process, and not much after that.
iPhone:
I'm currently using the iPhone 14 Pro, but have had nearly every iPhone and have used Lr extensively on my phone since it was available. I've noticed the same heating up, especially near the cameras, occasionally quite hot. This happens a lot when shooting HDR with Lr Camera and then waiting for the processing to occur. Again, while my phone has gotten quite hot, I've never had the screen dim as a result of Lr running hot. And the heat fades quickly once the processing is done.
I will add that I also live in Florida as noted by one responder above, and when I'm out in the sun with my iPhone, I have the dimming issue caused by the heat of the sun. This happens not with Lr, but with any app, even when I'm not using any apps and just looking at the Home Screen. This happens to me sometimes even if the phone is in my pocket, when I'm sitting in the direct sunlight. This is of course not related to Lr but shows how the iPhone can be sensitive to heat.
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