First, Practice. A lot. Try to do as many rounds as often as you can and the speed will come with familiarity. Familiarity is probably the single most important thing that can help you.The more familiar you are with your arguments, the more concise you will become which can help with speed.
Second, after every tournament, take your flows from every round that you lost, and give your rebuttals again and again until they are perfect. Turn them into rebuttals that would win you the round. This will help you learn your arguments better and it will help you learn to streamline important arguments so that you do not waste time.
Don't get discouraged and the speed will come.