In "The Emissary" we see an early portion of the Battle of Wolf-359. The U.S.S. Melbourne approaches the Borg cube and is effectively destroyed in a single shot by what looks like a combination of the Borg tractor beam and cutting beam. Then it turns it attention on the U.S.S. Saratoga. It locks a tractor beam on Saratoga and starts training its shields. Once the shields fail it fires its cutting beam and mortally wounds the ship, causing it to begin progressing to a warp core breach. This takes several minutes allowing the surviving crew to evacuate. The Borg cube has plenty of power for its weapons as while Sisko is trying to free Jennifer we see the Borg dispatch an Oberth class ship with a single shot. Then once the crew is evacuated, with the Saratoga about to explode ANYWAY, the Borg cube finishes it off with a single shot? Why does the Borg ship behave like this? Don't say for dramatic reasons (i.e. to allow Sisko and Jake to escape) because if the Borg had simply severely damaged the Saratoga with a single shot causing its warp core to progress toward breach then we would've gotten the exact same race against time.