My wife seems to have a problem where charging seems really slow, luckily its insane battery life means she is surviving but it is going to run out eventually. Going to try the official charger and cable to see if that makes a difference
Being a large battery, if using a laptop or low power USB mains adapter it will charge slowly. The supplied USB charger is 2 amps, 4 times more than most normal USB ports will deliver.
I've upgraded my car chargers and mains chargers to make sure the phone fast charges, and the upside is all my other stuff now we charges quicker..