A phone can be put at risk if it gets too hot - components can be damaged, screens can crack and batteries may even catch fire, causing the handset to explode. To protect against this, modern smartphones have inbuilt response systems that stop a phone working too hard when it approaches dangerous levels of heat. To begin with, they will start to slow down the processors, which is why your phone might start to slow down in hot conditions: it’s not anything wrong with the phone, it’s just that it’s entered self-protection mode temporarily.