Once you attack the city it's all instanced, yes. Unlocking the city for attack doesn't really take much. Lock down 2 of the 3 Tier 4 racial pairs completely, and the city opens. Since destruction usually owns the lower tier and usually beats our ass in scenarios, they usually have enough victory points that taking all the keeps and BOs is enough to lock the zone. Now to do this fast, obviously the more people the better, but if there's no resistance it's not going to matter.
People say it takes to long now and the next patch is actually going to make it shorter, but I've been on the receiving end of a mad rush like this when we were outnumbered in RvR 2 to 1 and they took a zone with 100 of us defending in about 3 hours. We just couldn't recap any of the BOs long enough to slow down the victory point gain.