Could be Apps running in the background.
What you can do is to check for suspicious apps and also do a spring cleaning of the apps you do not use by disabling them.
if you updated /flashed to a major build of android such as from Android 5 to 6, it is best do a factory restore.
Lastly it is good to do a restore every now and then.
If it does not work out, send it to the service centre at 313 for a peace of mind
I Play games on my phone with data and WiFi. For like few hrs or mins. The thing is getting Warmer and warmer. Was it a good or bad sign? Will the speed performance degrade after 2 yrs?
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