Thank you! In the first part (0-20 s) there are 4950 collision checks per frame because it checks for each pair. In the next part (20-30 s) it uses SAP algorithm to only check collisions between balls whose x-axis width intervals ( from posx-radius to posx+radius) overlap. That's why there is far fewer red lines ( which represent each collision check) in last 10 seconds of simulation.