I'm also having this problem. Seems that restart helps a bit for some time but not for long. It is really an awful experience to use the phone like this. Having the 4 GB version.
Do you see the drain during use or when the phone is in standby?
I have good battery life in standby, using Greenify that is possibly helping. But during use it is draining fast :/ Have not found any solution so far.
What's your screen on time before and now? I got worse both standby and activate. A couple of other guys in a swedish forum also got much worse battery life.
I got much worse screen time with Oreo update. Did a full reset but still seeing the same numbers. Using AccuBattery and it shows 1% battery for about 1-2 minutes of screen time.