Does it only charge slowly when it is turned on or also when off?
If it is both ways, then your battery is probably on its way to needing replacement.
If it charges fine when off, but not when on, then I would start looking at your "Settings" as well as what apps are running. Some apps use a lot of power, causing the charging to slow or even seem to stop.
You might want to try starting up the phone in "Safe Mode". You won't be able to access most apps in it, but you can see if the battery charges faster. If it does, then you definitely have an app (or virus or malware) on the device that is causing the drain/slowness.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power"...