When you update your phone to Android 6.0. You will need to do a Cache wipe and factory reset. Restore device as new device. Do not setup up SD card as internal storage just yet. Give the system a couple of days to settle in. Make sure to update all your apps.
Charge your phone to 100% then use it normally. I’ve noticeda big difference in performance. No heating issues when I leave my adapative brightness on. Marshmallow has bring this phone back to life. I use this more than my moto g3. I think this moto g2 is better in my opinion.