Good afternoon,
It appears that the A12 is capable of 15W charging as well as sporting a 5000mAh battery, while not slow by any means this combination is not the fastest at its best.
I would first ensure that you are using the stock charger, if you are using an aftermarket charger look on the documentation or the model info printed onto the block itself.
If it does not read 15W I would suggest replacing it.
If you ARE using the the stock charger and said charger is not malfunctioning / or degragated I would then look into the phone itself.
Start by reading up on this [post|382427|comment thread posted a while back ago|new_window=true], as you might want to try to clean out the port
If cleaning the port is not sufficient you might want to look into 1) [guide|154307|replacing the port *i.e sub board|new_window=true] or 2) [guide|160187|replacing battery|new_window=true]
I hope I was able to give some assistance.
I understand repairing a device can be over whelming, however there are plenty of people on iFixit that all love to leand a helping hand, please do not be afraid to ask for assistance.
-Lucas