I believe that my USB 3.0 port provides less voltage, but more ma than the wall wart. I'm using this as a clue, but maybe I'm using it incorrectly:
The current rating of a wall wart is only what it can supply not what it will supply. That is governed only by ohms law. If it is the same voltage it is the same current.
What could change is the grounding, connected to a computer you will have better grounding which will make your marginal signal better.
The capacitance from the long wire will slow down the rise time of your signal, a lower resistor will speed this up.