That was my conclusion as well, the only arguments might be at certain speeds the distance per move might be such that you would get slightly more if the gold/seeds gained is rounded after every move rather than just a hidden number thats rounded at the end of the map. Then there's potential min/maxing if the rounding is 'always up / always down / nearest whole'. If the rounding exists at all, its also possible that any remainder is just dropped.

So I'm fairly confident to say that slower units don't increase net gains, but there may be a 'sweet spot' for speed to sit, but determining that value is definitely beyond my grasp, and the gains seem like they would be pretty inconsequential.