If everything is slow, then whatever is causing that is probably causing the power drain.
The things I would try are:
- Restart the device.
- Check if the database is corrupt. If you use calibre, you can do this with my Kobo Utilities plugin. Otherwise, you need an SQLite database management program.
- Reapply the firmware. There are occasionally some weird problems after a firmware upgrade. In a lot of cases, reinstalling the firmware fixes them.
- Do a factory reset. When I do this for problems, I take a full copy of the user partition, do the factory reset and test things. Then I decide whether to restore the files, or just continue from where I am.
And if none of that helps, you can do a factory reset and go back to 3.12. Or try one of the versions iin between.