Call the captains over to the stand when this occurs and give them a verbal warning, that they need to hurry with their cheers and get ready to serve or receive more quickly, tell them that if they continue to take too long after each rally, you will assess them with a team delay which will cost them a point/loss of rally. That should fix the problem. I suggest that you do it early in the game when it first begins to happen, that will save you some time.
