i measured power-draw at the wall outlet:
Just the Duet: 9W
Starting bed heating: 320W
Later on, at 60C: 280W.
So the bed heater uses around 310W at the beginning and 270W later on.
Seems about correct.
So i don't know what i measured or remembered from before. I guess i will live with the heat-up times for now. At least i am finished with ABS printing for now. Too much warping even on small parts (7x7x8cm) without a full enclosure. I will go back to PETG, so heating times will not be soooo bad.
THX for all your input!