It needs practice to get comfortable speaking slowly. Pick one friend, tell him you are going to focus on speaking slowly and practice it for one conversation. Speak exaggerated slowly and limit yourself to a few words per breath. Then pick another friend / situation and practice again and slowly expand your comfort zone this way. I myself am currently very comfortable in speaking this way in one-on-one coversations, but find it more difficult when speaking in groups. I feel there is more pressure to speak fast in groups, so have to work on getting comfortable there as well.