Everything drops at the same speed. A good BC will help a bullet retain velocity so it'll get further down range before it hits the ground but it still drops at the same rate. If both bullets have the same BC but ones heavier than the other, as long as velocity is equal they'll both hit the same. A heavier bullet, or arrow usually goes slower & hits lower as it wont get as far as a lighter, faster bullet or arrow.

The way it was explained to me is, If you were to drop a bullet at the same instant you fired one from a level barrel, they'd both hit the ground at the same instant. Just that one would be at your feet & one would be downrange somewhere.

Theres alot of variables in an arrow, fletching, design of point ect that affect how fast it slows down so it may not be as cut & dry as a bullet but I'd think the physics are very similar if not the same.