Updated over the air from the last developer beta on an iPhone 11 and on an iPad Mini 5; overall, battery life and performance have been pretty excellent on both devices post-installation. Keep in mind: Reduced Motion/Reduced Transparency are enabled and Siri is all but disabled through Screen Time, which of course is also disabled
P.S.: If any of you is curious or wants to know more, I also disable all options under Privacy > Analytics & Improvements, as well as enable ad-tracking limitations under Advertising; additionally, I use a content blocker—AdGuard Pro to be exact—in Safari. On both devices I have True Tone, Auto-Brightness, as well as timed/automatic Night Shift and “dark mode” enabled.
On the iPhone 11 I have found a happy-ish medium of security and battery life by enabling Face ID with Attention-Aware features disabled (I know, risky from a security standpoint. Eek). On the iPad I just use Touch ID normally.
On both devices Fitness Tracking is disabled under Privacy > Motion & Fitness and generally I run a rather tight ship when it comes to granting location access/permissions to Apple and to third parties (e.g. I don’t believe any app is allowed to “Always” access my device’s location). Here’s a glimpse into my iPhone’s Location > System Services settings:
Keep in mind all settings under “Product Improvement” on this page are also disabled.
One final tidbit:
Due to health, privacy, and battery efficiency reasons I use my cellular-capable device(s) (in this case my phone) with wifi/Wi-Fi Calling enabled—as needed—and with cellular and bluetooth enabled/disabled as needed, but preferably off. Although this may not sound suitable for/suited to everyone’s needs or lifestyle, I’ve found that it’s worth some experimentation to try and limit one’s exposure to potentially-harmful electromagnetic radiation. And if one doesn’t care, doesn’t find it feasible or worthwhile or just isn’t interested in this practice or science then so be it; to each their own. Either way, one thing is certain: Disabling radios when unneeded/not in use is easier on a device’s battery.