iPhone 15 became very hot After iOS 26.5

When did you update?

You should expect heat, lag, and increased battery usage immediately after the update and that continues for about 4 days. There are many things happening after the update including the files being indexed in the background and photos are being optimized with any updates to object detection algorithms. That is documented by Apple here:

About Apple software updates - Apple Support

If it has been longer than 4 days since the update, then check for your Battery Health percentage at Settings > Battery > Battery Health. If it is below 80%, it needs replacing.

“iPhone 15 became very hot After iOS 26.5

After upgrading to iOS 26.5, the iPhone 15(model:A3092) became very hot and the battery life decreased significantly. iOS 26.4.2 is very good. What is this problem?”

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Overheating iPhone:

It may be trying to contemplate the new specs. So, try cooling it down, when not in use. Use my User Tip: Cooling Down an Overheating iPhone - Apple Community

You may want to update to iOS 26.5.

“It's been a week already after update, but still haven't improved.”

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Overheating Battery:

Gotta say, my iPhone’s Battery Health is 82% —almost at 80%, which is when I’ll get a new battery. I'll have Apple replace the battery, to avoid all the fraudsters placing non-genuine batteries in there(which reported oh-so-commonly on these forums). Using an Otter Box, it doesn’t feel hot in my pocket, unless I press down on the back a little bit. So, use an OtterBox, if having it with you is a must.

ZXONMARS wrote:

It's been a week already after update, but still haven't improved.

And your battery health is ....?