The only thing I was ever able to do to get my iPad Pros to stop when the standby drain would get excessive was put them in airplane mode, essentially turning off all the power-sucking radios and cutting off access to iCloud. This reduced standby drain to almost nothing--resembling what it used to be like in the old days. There was never anything excessive running in the background according to the battery reports, DFU restores didn't fix it, unit exchanges didn't fix it, etc. I spent far more time troubleshooting this specific issue than I'm usually willing to because it was really annoying and really deprecating my iPad usage experience.
I just don't think the current iPad form factor can contain the battery it takes to get standby time similar to what the Mac is getting. I'm not an engineer or anything like that--it's just a guess. On the flip side though (forgive me if I've already mentioned this) I never turn wifi or cellular or anything else off on my mini 6, and I think it does far better than the Pros I've had in standby regardless.
Airplane mode improves it, but it is still far worse. I mentioned earlier that my iPhone Xʀ running iOS 12 isn’t good on standby: I recently left it for nearly 24 hours in Airplane Mode, with about 30 minutes of screen-on time throughout (which isn’t even enough to make it drop from 100%), and after 22 hours it had… 92%. So it lost at least 7% in one day of standby, with perfect conditions: optimized settings; Airplane Mode, and the original iOS version (12). My iPad Air 5 on iPadOS 15 (its original iOS version), drops 2 or 3% overnight, or a little more, after 9-10 hours. That is not good. 8% per day? For an iPad? It should drop 1% at most, if that.
So even full optimization - at least in my experience - isn’t enough to get good standby drain. Something is horribly worse, because if I put this much care into trying to get a half-decent standby, imagine what someone who just enables everything gets: the result would be abhorrent. What, 20% per day? More? It’s just not good, and people aren’t mentioning this.
Battery capacity might be a possible reason, but the reason falls apart when you look at iPhones: My iPhone 6s on iOS 10 has 63% capacity, about 1080 mAh. My Xʀ, when new, had nearly 3K mAh. So triple the battery capacity. I can assure you that my 6s on iOS 10 obliterates the Xʀ even with the latter on iOS 12.
For iPads, maybe iPadOS 13 was the issue, but somewhere around iOS 12 or iPadOS 13, something changed, for the worse. With all of the efficiency gains Apple has shown on hardware, it is really a wasted opportunity to see current flagships (both iPhones and iPads) on original iOS versions be obliterated by 7-year-old devices. Something broke in terms of standby, and there’s nothing the user can do to fix it. Software optimization through disabled settings and Airplane Mode only goes so far, the OS just does more on standby even with everything disabled, and only Apple can change that.
I mean, I’d call 8% per day quite abhorrent on a phone with a 3K mAh battery, but that’s just me. Why do I call it abhorrent? Because older devices were far better. My iPhone 6s on iOS 10 would probably need 4 days or more to drop to 92% on standby, even with a battery that’s one-third the Xʀ’s size.
As far as iPads go, another comment mentioned that they lose about 6% per day. Give or take some variation, we are talking about a similar drop as my iPhone. It should be better. It should be at most 1% every 24 hours. That’s how it used to be.
