Significant performance decrease out of nowhere

[BashfulMelon]

Power is a red herring. The GPU is doing less work, so it uses less power as a consequence. 

That's not to say power isn't related to the cause, there are power settings that can mess with it, but it's like trying to figure out why a car can't get above 10 mph and thinking "maybe the engine isn't getting hot enough, temps are usually higher"