For an RCL 8 base the defender loses maybe 15% of the energy into the controller. Ticking down an RCL 8 to RCL 1 takes significantly longer than 6 ticks.
Consider this: 15% of an RCL8 controller equals to 1.6M energy, or 160K ticks per source harvesting. This is the time which the owner has lost even if he managed to recover.
you need to regain power in a room for 6 ticks to undo that seiging
Not exactly true. The defender needs to fight back and regain control over the controller for 1000 ticks in order to execute those 6 ticks. The 1000 ticks is the crux, not the 6 ticks. And the attacker is supposed to guard the attacked controller while he's downgrading it. If your tactic is sending an one-time raiding party and then just simply sending a lone claimer every 1000 ticks, you're not supposed to success.