Agree with QuintLeo. And just to add...

I have used some cheap-ish "pure Molex" risers which were making 3.3V from that 5V rail coming from "the Molex". 3.3V power line should not see much load if any. I measured it on a couple different GPUs and it was well under 1A, which is less than 0.3W.

Those cheap step-down LDO voltage regulators they put on risers still have decent efficiency at low currents. Say, 80-90% if not more. Do the math and get 0.3W - 85% = 45 mW of power loss on the riser!