I have a nexus 6P with a fairly new battery (some months). It worked quite well, until I decided to flash Pixel Experience because it is the only ROM I found which supports the 6P with Android P. After flashing, the phone suddenly started draining its battery extremely fast (1h screen time, ~4hs idle). I couldn't find any troubleshooting for that special scenario, do you have any ideas on what to do? I would prefer to try everything before changing ROM because I really like to have Android P.
Have you tried Greenify? It's on Google Play Store. Install it and grant root permissions. You can read about it here: https://forum.xda-developers.com/apps/greenify
You can also try to flash Derp Kernel