I just did the factory reset, and I noticed the differences that you mentioned when I booted for the first time after reset. Now I'm doing a restore using LG Backup & Restore. I've had great success with that in the past - with G3. I just hope it won't restore the "bad" stuff that may have caused the poor battery problem.
Update: It's been a few days after the factory reset and performance is somewhat improved but not close to where it was with Lollipop. The factory reset was painless - so not much downside. This is what I did:
1. Used LG Backup and Restore to save the backup to the sd card. I also saved a copy to my PC...just in case.
2. System settings->Backup and reset->Factory data reset
3. When rebooted I let Android do it's thing to restore. (the first time I stopped it in the middle and skipped to the next step but the results were not good so I did it a second time, and let it finish. The Marshmallow restore is improved!)
4.Opened LG Backup & restore and let it find the available saved backups. I chose the latest one to restore.
Everything was close to 100% back to how it was before reset. Some text messages - conversations from prior years were out of order - a handful appear on top.
The only annoying thing - the default sms app changed the setting back to "Delete old messages" - lucky I caught it and no messages were deleted.
Smart cards are appearing occasionally for some reason, although turned off in settings.