Surely, that's quite a huge drain. There could be so many culprits though.
Pixen-OS is no longer updated and support is dropped btw. I suggest switching to another ROM if u want support and updates. Otherwise, by all means, carry on enjoying the ROM
OTA When an update is available, you’ll get prompt for it. From the Updater, Just Download it , Install and Reboot. * After download it just takes 15 minutes if encrypted and 10mins if decrypted all happening in *Background You can use your phone during this. As simple as that. If you are rooted, root will be retained. No worries. For flawless updates to work on decrypted system, make sure you are decrypted on both slots.