You should read the documentation.
"Use iOS and iPadOS devices where the ambient temperature is between 0º and 35º C (32º to 95º F).
Low- or high-temperature conditions might cause your device to change its behavior to regulate its temperature. Using an iOS or iPadOS device in very hot conditions can permanently shorten battery life."
Learn about the operating temperatures and temperature management of iPhone and iPad.
support.apple.com
Computers and batteries like to operate around 20º C (68º F) ambient temperature. The more you deviate from this, the more likely you'll experience suboptimal performance.
Also a lot of cases aren't good at heat dissipation.