Does a Linebacker have to be fast?

I asked my future coach if I could become a Middle linebacker and that I was at bit slower than average speed at my school (5.3-5.7). He said:"No problem, we can always train on your speed." But I am not quite sure. So does a Middle linebacker have to be very fast? I am 13 years old if people want to know that and I am 5'9'' and I am not sure how much I weigh in pounds but I weigh 74 kg.