Analysis
The person speaks very fast, but it is easy to listen because he uses the emphasis effectively.
He explains the detail of the content very fast, then asks a question. He pauses a little, then emphasize the part "but why..." so we can tell which part we should pay attention.
When he answers the question and makes the point, he explains a little slow, and emphasize each word. In the summary of his argument, he uses louder voice and higher tone to attract the audience to tell them he is saying an important thing.