Pay attention to audience feedback. If one person reports a problem with understanding you, this may be an individual perception or opinion. But if several do, it's time to time yourself. 

Try this test. First, tape-record a casual conversation with a friend. Then compare the number of words per minute to a tape recording of one of your recent speaking presentations. Do you always speak quickly? Or just when you're giving a speech? Was your presentation deliberately speeded up to meet some time constraint? If so, were you trying to include too much material? (That's a signal to cut some information so the rest is more effective.) 

If you decide you need to slow down your delivery, start before you even hit the podium. As you're putting together your remarks, think about logical places to slow down. It's okay to speak quickly as long as you leave yourself room for pauses and silence. The faster you talk, the longer your pauses should be. Give the audience time to digest what you've just said. If you say something really profound or suggest something like, "Consider the proposal in front of you," you are asking the audience to think. Give them time to do so.