Ok I upgraded to TWRP 3.1.0-1 and it changed some things regarding the charging current:
Charging current while the system is on: 550-680 mA (double the value in comparison to 3.1.0-0)
Charging current while system is off: 700-900 mA (ramps up to 900 mA and then drops to 700 mA)
Definitely changed to the positive side, system performance while connected to the charger has improved quite a lot, as well.
On the lock screen it displays "Slowly charging" (translated by myself, i've set it to german).
I think I discovered a bug:
When I try to start the system while it is connected to the power plug, it does not start.
It turns the screen on and off, showing an empty battery symbol with a loading symbol placed central in it.
When I unplug the tablet and then try to start, it starts flawlessly.