What causes the low fps?

Here is an easy explanation

Gamecube and PC doesn' speak the same language and doesn't have the same architecture (RISC vs SISC)

so you need to convert from gamecube language to PC language, and Do the same with the architecture...

That s why it takes time to translate....

This applies to all emulated consoles really. They all start quite slow, but gradually get faster with improvements to the emulator to convert the consoles language to PC language as Kawoul correctly says, and also with improving PC hardware.

It all depends on how complex the system is too. Even the PS2 (despite being less powerful than a GC) is still complex because it has limited documentation as to how it works, and slow because the hardware is very different to a PC as it has many co-processors that work together. Same sort of concept applies to the GC.

never heard of SICS, guess you rather meant CISC (Complex instruction set computing).
And it really doesn't depend on that, ultimately it somehow does but rather on the different architecture, and even if it, the architecture, would be completely coherent except a different opcode-look up table you would have to employ an interpreter.
BTW: all the PCs we are currently using are too RISC machines although running on a CISC set withhin the architectural interpreter, that is the CPU interpreter.
The real work starts once you try to emulate and especially simulate a whole system.

BTW: just awesome what the dolphin team created!!

Lol you are right
It CISC