It's a very common reaction, particularly at the Supreme Court, whether the lawyer is French-speaking or English-speaking. If it's their first appearance, they're generally nervous. When a person is nervous, they frequently speak a lot faster than they intend to. It would happen very frequently that you would ask the lawyer to slow down. We use the excuse that the translators need time to translate, or the clerk needs time, to try to relax them. Sometimes they hyperventilate. We call an adjournment and let them get a drink of water.
Your first trip to the Supreme Court is a very trying experience, so French or English makes no difference. People will frequently start to speak so rapidly, and there are usually more English provinces' cases just by virtue of the number, so it's more frequent that the advice is given to an English-speaking lawyer.