I have several WvW characters, so here goes (big breath)
1) No supply mastery—-picks up supply slowly (as expected)
2) First rank in supply mastery—Pick up supply faster (as per description)
3) Full supply mastery— Picks up supply agonizingly slowly, far slower than with no ranks in supply mastery, in some instances I can be mashing the F button on a full supply and still not pick up any (no lag at the time). Standing directly on top of the supply or edging slowly out has little effect. Unknown if this is due to character behavior causing this, unlikely but should be mentioned (asura/nec).