How to Downgrade Your Nexus 6P
If you can't stand the Android 8.1 update's performance on your Nexus 6P you can try dropping down to an older version of Android Oreo (or Android Nougat if you really can't stand Oreo) to see if that improves things.
If you’re unfamiliar with how the Android downgrade process for Nexus devices works, take a look at this guide courtesy of the Nexus Help Forums.
The walkthrough will take you through the steps you need to take in order to move off the current version of Android you're currently running.
There's no guarantee downgrading to Android 8.0 or Nougat will help, but it's worth a shot if Android 8.1 is wrecking havoc on your device's performance.