This. Big batteries need the beefy 5.2V-2.4A (12w) that comes with the iPad. If the 6+ has a large battery, the usual lower power chargers will take a lot of time to get you there, and you'll find out that the charge despite being supposedly 100%, lasts shorter.
On vacation I took just one charger which looked "big enough" (as big as the original iPad one) but I noticed it needed a whole night to charge the iPad just to 90%. After checking it came out that it was just an 1.5A charger That could be your problem CJ.
Now the full disclosure, I know shit about the 6+ so it's possible you got a defective battery in yours.