Since your phone is becoming warm, which implies phone activity - a common issue in Windows 10 Mobile technical preview - you may want to check the output of your phone charger; it's highly probable that the rate of power consumption of the phone is higher or near the output of your charger.
Another way to determine which is consuming most of the power in your phone is to disable some of the features, such as disabling location services (disabling this has been reported by many users to save power and battery life) and activating airplane mode.