Safe Mode is one of troubleshooting step that recommended for analyze the problem on your Nexus 6P. It can give you the information whether the problem is caused by the third party app or not. Actually Google is attempting to prevent the problem with Android 6.0, but some apps still can distract your device.
That distraction can make your phone battery drains quickly, heated up, or other problems disturb your phone usage. So, it is important to know how to boot your Nexus 6P into Safe Mode and use it to fix your problem.
You can boot your Nexus 6P into Safe Mode while the phone is ON. Then please press and hold the Power button until you see Power Off dialog screen pop up. As soon as you see it, please let go off the Power button then tap and hold on the Power Off option for a view seconds. You will be asked whether you want to Reboot into Safe Mode or not, and tap OK to reboot the phone into Safe Mode. Please wait until the process is completed.
After you are in the Safe Mode, you cannot access the third party apps and you can use your phone normally with just some basic apps running. You can remain using this Safe Mode on and monitor if your problem is actually resolved by this safe Mode.
You can compare the usage of battery in this Safe Mode and while you are not to analyze whether the problem is caused by the third party apps and resolve your battery draining problem.
If the problem is overheating on the device, you also can compare the temperature changing in this Safe Mode and while you are not to analyze whether the problem is coming from the third party apps. You also can compare the performance of your phone like the lag issue or slow loading problems on your phone by monitor them at the safe Mode on and while it out of the Safe Mode.
You can exit from the Safe Mode simply after you done with it. Please hold down the Power button and turn off your Nexus 6P. After it turned down completely, please turn it back ON like how you turn on your device normally. Your device will back to the Home Screen normally and you already exit from the Safe Mode easily in a short time.
After this step, you will take some actions to uninstall the suspicious apps on your phone if you find it on the comparison of the Safe Mode and not Safe Mode performance on your Nexus 6P.
However, if you are not see any difference on the comparison of the Safe Mode and not Safe Mode performance on your Nexus 6P, there is a possibility that the problem is caused by your device software. You can try to wipe the cache partition or factory reset on your Nexus 6P and see whether the problem is resolved.