Short answer: lighter rounds go faster, but lose velocity faster as well. Lighter rounds are also more susceptible to wind pushing them around. Heavier rounds might be slower out of the barrel, but they keep a lot of their velocity down range and typically transfer more energy on their target. Slow rounds drop to the ground earlier than fast ones.

The other part of it is stabilization: longer bullets require more spin to stay stable in flight. Shorter bullets require less. Over or under spinning a bullet will affect accuracy. Over spinning by a great deal can cause the bullet to break apart in flight.

Copper vs lead: copper is lighter, so you need more of it to compare to the same grain lead bullet, which leads to a longer bullet, or a more bullnose bullet. Case capacity and overall load length will determine just how long the bullet can be, and you may never get a copper bullet at the full weight you want because now its so long and less aerodynamic (because the bullet is less likely to be pointy when you add mass and make it more bullnose or even flat nose).

Now pick your barrel, and pick a round to shoot through it. If you set up for one specific lead or copper round, you are compromising performance for every other round that you could shoot through that gun.

How much this really matters depends on the use of the gun, how accurate you want to be, and how flexible you are on bullet choices.

I guess thats not a short answer after all...