Really need more context to fully answer the question, but, assuming you are talking about the start of a sentence: "The speed of" would normally be followed by an object (e.g. sound, light, horses, thought, etc.) "A speed of" would normally be followed by a given speed (e.g. 200 kilometers per hour, 5 centimeters per second). Said differently, the topic of a sentence which begins "the speed of" is an object, while the topic of a sentence which begins "a speed of" is speed.