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[Star Wars: Rogue One] Two ISDs couldn't take down the Rebel Fleet, but Darth Vader's Could?

During the fleet battle, we see that there were two Star Destroyers engaging the Rebel Fleet. One was eventually disabled by Gold Squadron which led to the other being destroyed.

However, when Vader enters the battle his ISD obliterates the fleet with impunity. 

Why the difference?
gpt
Scarif was a pretty cushy posting, and a lot of officers (especially higher up the chain of command) were pretty lax in enforcing proper training and preparedness - there was zero expectation of experiencing any actual combat, much less a full on fleet action and ambush.

So the Star Destroyers were caught off guard, out of position, and with crews asleep at their stations. That said, they did do a number on the Rebel fleet before their destruction.

Then in comes Vader and the Devastator. His crew are already among the best of the best, they're a well oiled fighting machine, and they're coming in hot and ready to jump into combat. The Rebel fleet has already taken a pounding and is preparing to fall back, and their fighter/bomber wings have already dropped their ordinance on the prior Star Destroyers and the Shield Station.

Vader's really just coming in as a cleanup crew, and if the local Navy detachment were doing their jobs properly, would never have been necessary in the first place.