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restore the software? there's a lot to point to but with no information of what apps you use, background activity, locations & what decreased performance you're referring to.

mine and my wifes iP6 (not plus) running on iOS 10 has been working perfect. no issues.
 
Try reset all settings. You won't lose data, but will have to reconfigure the device and settings to your liking. If that doesn't work, a restore as new might be in order.
 
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