My iphone 6 become hot after updated to the latest ios 11.2.1

my iphone6 become incredibly hot after i updated. whenever i watch youtube or playing games for less than 10mins. the phone starts to became hot and everything became slow. i cleared all running apps, switched off background app refresh. i still have more than 20gb space. but still it didnt help much. my phone still slow and sometimes the application exit by itself. i have turned off my phone and switched it on back after 10mins. the phone still slow. please help and advice