My buddy just built his first open loop with Corsair and called me with the same issue. He says the flow is terrible and he has giant air pockets all over.
To be safe on initial fill and bleed he is running just the psu to the pump and using the psu jumper to power the pump via molex. So for the OP to have a weak pump if not ran through commander and my buddy to have the same either they are shipping faulty pumps or something has been changed in them.
I personally use EK pumps/res and the things will suck a 200ml res down like nothing just powered off a jumped psu. So this whole weak sauce pump issue is all new to me and had to go looking. Nice to know he isn't the only one having this issue.