Yes, I said the GPU is 10 times faster, exactly what I was looking for. But only for Superfly.

For Firefly, the CPU is not that much slower, but image is not as good. But it takes only 420 sec.

So if the image does not fit into GPU memory, than you are stuck with CPU rendering. Above image used 5.5GB of Vram. If you have a card with only 4GB, then you cannot render in Superfly, not sure. I know in Blender, it will kick you out of GPU render.