I'm not real clear on your question. I'm having difficulty even understanding what you are talking about.
The arm doesn't move at a constant rate of speed during the pitching motion.
The arm moves fast from 12 o'clock to release. It doesn't move fast as it moves to the 12 o'clock position. So, she adjust how long it takes for her to move her arm to 12 o'clock. She then pulls down with her elbow to start the downward motion when her left foot is a couple of inches off the ground.
Can you rephrase your questions?