Classic
emulators have to emulate everything (i.e. processor, RAM, ROM, video processor,
keys, timers, ...) with only the use of the computer processor, that is why you
need at least a 700Mhz computer to emulate a small gba (which has a 16MHz processor).
The GP32 processor is compatible with the GBA one, so the first idea is to not
emulate the gba processor but to directly run the code present in the GBA roms.
Of course it is not as simple as to rename a .gba file to a .gxb file, but it
runs !