The other side of the coin is that cup in core bullets may kill faster precisely because they lose mass and expand with large frontal areas that limit penetration.
Assuming that they penetrate enough to reach vital organs,the loss of mass creates secondary wounding and the large frontal area allows for very rapid energy transfer. These two effects incease the volume of the wound channel even if the length of the wound channel is smaller.
If you shoot the same bullet in a 7x57 and a 7mm Magnum,the Magnum cartridge may be going 500 fps faster at impact. The 7x57 may produce the longer wound channel,but the 7mm Magnum will produce the wound channel with the greatest volume.
So depending upon circumstances,either cartridge may be the quickest killer,but generally speaking the biggest(largest volume) would channel kills the quickest.
