Oddly enough, when I use my iPad charger my 6S seems to charge even faster than normally. There has to be a problem with the cable, charger or phone for it to take that long.
As has already been suggested, a visit to an Apple Store Genius Bar is warranted. It's impossible to pinpoint the cause of your problem without a full diagnostic. Make sure to backup your iPhone before taking it in.
Hi you have been be given some great advice, by people that know what they are talking about.
They cannot repair your iPhone. Apple can. Or if it's out of warranty give you a quote for repairing it, try them you never know. Good luck