I would suggest speaking to your instructor about what the applications of each move would be. I am sure you realize that it is hard to do tai chi very fast, but it is also very hard to do the form very slow. Medium speed is what people do mostly. Keep in mind also that the speed must also be measured in parts. The entire form should be evenly paced.
Most people start out slow, and then speed up when there is a strike or retreat.
Try to be consistent. If you intend to make this a part of your routine, I would do so sparingly, until you are extremely comfortable with a slow to medium speed. However, I think once you try it you will feel your weak points, and will then be able to concentrate on those when doing it slow. After you have tried the whole form once fast, try then to separate sections. Perfect each section, and then try to do the whole form once again.