I can see why you think that, but just think about it a little, the GPU on a gamecube is from 2001, dont you think that the GPUs today are more powerful, even if you dont have the top of the line, even a 5200 is more powerful than a gamecube graphics card.
Anyways, what Im saying, is that the GPU is not what slows the dolphin down, its the CPU. The arcetecture of the GC and a normal PC are quite different, and trying to emulate codes to from the GC is very time consuming. Because it has to process like a GC and these aren't simple conversions. These conversions are like converting whole commands, processing them, and converting back, and takes alot longer than it sounds. And this is alot more complicated than it sounds.