My cell phone seems to charge slowly, especially at my home (I've tried different outlets). But recently I was in a coffee bar, and it seemed to charge a lot - a lot! - faster. I was using the same charger. (My phone is an inexpensive Samsung Galaxy phone, I paid like $50 for it.)
Could be because
Coffee bar may be using charging stations which may look like regular charging wall outlets but seem to be designed to charge faster (have not been able to get technical view to back that yet and am searching)
Outlet at home is not providing sufficient wattage either in terms of voltage or amperage, may be because of poor electrical earthing
There may be some issues with the input and output voltage with your phone, or its charger. I strongly suggest getting a new charger. I just got a new charger from Amazon. I have a
Samsung Galaxy S4, but I bought a charger for the
Samsung Galaxy 6 Edge. This charger is a lot faster, and is consistent with charging speeds.
It depends on the voltage and current being supplied to the charger at different places. Like for me while travelling via train, the sockets have half the voltage but higher current(Amps) which used to charge my cell faster(of course cannot exceed the maximum output of your charger). I guess so, can be some other reason too.