The article linked by BugeyeSTI explains it:
Discover why your iPhone 16 Pro Max feels hot despite new cooling features. Learn about thermal management and performance trade-offs.
www.macobserver.com
Basically, no matter how heat dissipation is improved
inside the phone, the heat needs to escape outside the phone to the surrounding air which it can only do by going through the phone casing. So if heat is pulled away from the processor/etc more efficiently, it will improve performance but will result in the user feeling more heat on their hand. Because the iPhone uses the casing for heat dissipation, the only way to for the user to
feel less heat is to insulate the phone (which is bad for the phone and performance), or for the processor/etc. to generate less heat in the first place.