The A12 with 7nm CPU, running at a slightly lower speed in order to keep the GeekBench4 score at least the same as the iPhone X/8, could likely have pushed the Xs battery performance past most Android phones, and much further than the 2017 iPhone X. Maybe that could allow us to have enough power to surf the web and post to facebook or watch movies for longer periods of time.
On the other hand, maybe the low-power cores are already handling this for us. One way to be sure would be to set the iPhone into low-power mode and then use it constantly to surf the web and read email, and then see if it lasts longer than in full-speed mode after we have already turned off background app refresh and push email.
I still feel like we should have more options to extend battery life than just low-power mode, such as giving us a "mid-power" mode that lets us throttle the CPU but without turning off background app refresh and email fetching. However, with all of the low power cores, maybe it's already doing that?
I really need to test my Xs max and see how it compares to what my X has been like for the past year. i tend to drop my phones onto a wireless charger when I'm sitting in the family room, or plug into the car stereo when I go for a drive, and that keeps them topped off. In 10 days I haven't gone full unplugged all day with the Xs max yet.
But on my iPhone X, with very little use (not in low power mode and with background app updates turned on for about 30% of my apps), I found that I can let it sit for up to 48 hours before the battery is dead. Using the iPhone X for mostly calls, texts, alerts, and emails, and my iPad for everything else, my X would have 20-40% battery left after being unplugged for 16 hours.
Reading a Kindle eBook with dark screen and white text for 4 hours during one of those moderate use days would drop the battery another 20%, so that I'd still have 5-20% battery left after a 16 hour day. Using facebook and youtube for 1-2 hours on top of that would ensure the battery was depleted completely within 15-16 hours. That's still about 2x as long as if I had just been watching movies or playing games.