How can a tone and speed of speech affect the message?

Fast speech can indicate urgency - for example, you would expect someone reacting to an emergency to issue commands with fast speech. People can use fast speech in any situation to create this same sense of urgency. Fast speech can also be used to say something that you don't necessarily want people to understand...radio advertisements sometimes have the legal terms and conditions spoken very quickly at the end of an advert.

Slow speech is the opposite - it can be used to relax people (think of hypnosis - usually, that is very slow) and is often used to make it easy for people to understand. You often see people speaking slowly when they are talking to people from different countries who don't speak the same language. Slow speech can also be used to condescend - you might talk really slowly to someone to indicate that you think they are stupid.