Why does the generation speed slow down when using the GGUF model with wan2.1?

I am testing the generation of wan2.1 with ComfyUI.

When generating with the following parameter values, is there a reason why the generation with the quantized model is dramatically slower and how can I solve it?

(Test settings) RTX4090, 856/856 pixel image, 14 steps, 2sec

①I2V 720 fp8 model (16GB)・・・ 160 sec

②I2V 720 Q5_0 GGUF model (12GB)・・・260 sec