The difference is in the body posture. If your back is relaxed and you have bad posture the result is you moving your shoulders from behind your hips to forward with the kick you then scrunch with the stomach movement.
Instead start from a front stance with your stomach pulled in your core muscles engaged and your torso directly over the hips. Moving the leg can be done with zero effort from your upper body and speed comes directly from moving the leg only engaging your hip flexor and thigh muscles. Try both ways several times and you will notice the difference.
