Oreo Battery life is atrocious on Nexus 5X. Do not upgrade if you can avoid it.
You cannot judge battery life after only a few days, specially after an update.
1. Do you charge it to 100% and live it alone until charge goes to 0%?
2. Do you unplug it in the morning and use it "normally" and see what charge will be left when you put it again on a charger when you go to sleep?
Different use cases drains battery at a different rate and its hard to use a new phone/a phone with a new OS "normally" because more time than usual will go into trying to understand it.
It may also happen that you start using the phone at a time when your phone usable is above your average.
But I won't decide I know for sure it's a problem for everybody, obviously (which doesn't mean either it isn't).
EDIT: but as philjohn mentioned, you can expect to use your phone more after an update (especially a major release) to try new features and because of renewed interest.