Try charging it while it is turned off. It should charge faster as there won't be anything running while it charges. Charging while it is on, even if you aren't doing anything on it, will take longer, especially if you leave any apps open, even in the background.
Now if you are trying to use it while charging it, then that will take an even longer time to charge.
If charging it while it is turned off doesn't increase the charge time, then I would look into using a different charger, or even needing to replace the battery.