Let the ROM settle and do couple of battery charging circles before you judge.
Last full charged 21hours ago, 6h and 23min SOT with 31% battery left. That means I will get 9-10hours SOT until the battery is empty. The usage was normal without gaming. The battery is the original ~3years old.
I can't find where is the problem with stock kernel really but anyway...
I did, I switched to Optimus Drunk after giving the stock kernel more than a week to prove itself. And given the fact that I had to charge it multiple times a day I'd say the amount of charging cycles was enough for it to "settle". Didn't improve at all. If anything it got worse as the time went on, until I flashed the other kernel.
The last time I charged my phone was roughly 2 days, 6 hours ago (according to BetterBatteryStats).
Since then I've streamed music on Spotify for ~1.5 hours (with the screen off) and used it for ~4.5 hours browsing reddit, watching YouTube, chatting with friends, a few phone calls and managing a bit of crypto.
I keep Bluetooth, WiFi, LTE, GPS on all the time. Location history and auto-sync are enabled at all times as well. The screen is always set to max brightness and I don't have any underclocks going on. So it's basically the worst case scenario. Just now did I get the 15 % battery alert.
I'd say that beats ~15 hours of standy time.