Most often the culprit of slow charging of your smartphone or tablet is charger. Despite the fact that the connector on the wire corresponds to the power socket on the mobile device, charging can provide less current than needed. Old phones enough, and powerful modern enough. Carefully read the output parameters of the charging. There should be an inscription on the current from 0.75 to 2 A.
The second problem is the cord. In the Internet you can find special browse USB-cables of different manufacturers on the same charger delivered to the smartphone from 0.5 to 2 A. Therefore, when slow charging, try to change the cord on the other.
The longer the cord, the greater the probability to obtain a smaller charging current. The optimal size is 1-1.5 meters.
Especially often slow to charge a smartphone found in the car. Try not to use cheap Chinese car charger. An absolute guarantee of fast charging can only be obtained with a proprietary device! At least in accordance to your smartphone has been tested by the manufacturer.