I see even iPhone X owners complaining about battery life after upgrading to iOS 12. After taking a look at the battery stats from these users, we can see a lot of background activity from Facebook, Whatsapp, and so on. Although I think Whatsapp needs background execution for "Whatsapp Web" functioning, I doubt someone needs Facebook background activity. In short: turn off the most possible (if possible all) background activity. I also turn off transparency and turn on "reduce motion" at accessibility options. If you're brave enough, turn off most notifications you can, but I doubt this will present too much difference, since I presume iOS concentrates notification fetching into a single service (which runs in background), but in this case it doesn't matter too much if it fetches one or a dozen of notifications.
Gimmicks will always eat more battery than not using them, doesn't matter how much efficient is your CPU/GPU.