On the three point stance, no need to have the arm so high and up at the start. It will only serve to tighten the shoulder and if you are being timed from first motion, you are being timed as your arm comes down and through yet you aren’t going anywhere until the hand gets past the knee. Instead, try starting with the elbow bent and the hand just behind the knee.
I got this from watching Number 2’s 40 yard dash video-fantastic resource!