Why is my ipad being charged at low speed and sometimes discharged even when plugged in?
If you charge from a standard USB port (on a PC) or an ordinary USB charger, it is quite normal to charge with up to 500 mA. This is the maximum you can get.

If you want to go beyond that, you'll need a USB 3.0 port or high speed chargers. For example Apple supplies special 10W, 12W, 18W etc. chargers for various iPads.

It is also normal that an iPad is capable of consuming more power than a 500 mA charger supplies. Therefore you can experience that the iPad is discharging the battery even when it is connected to a charger.

To avoid that, close power hungry apps (i.e. don't be playing an intensive 3D game while looking at your battery stats for example), and/or use a higher capacity charger.