Question

山田

On average, it took four milliseconds longer for the ball to reach the opponent

このitは何を指しているんですか？訳もよく分かりません🥲
to以下を指す仮主語？

前文は
After a number of experiments, it was discovered that widening the diameter of the ball by two millimeters would slow the ball to make a rally last longer.
です。