The Gamecube is a much more complex system that the PSX was by quite some margin. Emulating its 128bit 485MHz CPU is going to be quite taxing alone, plus the ATI graphics chip, and then for sound you then have to factor in its 64 channel 16bit DSP that supported Dolby, DTS and AC3.

It is a surpise they have managed to emulate the Gamecube and Wii at all.

I think the speed of the emulation is more down to the graphics card you have as well as the cpu and ram.