If you have very little time to prepare a speech, spend your time zeroing in on your purpose and your audience. Ask yourself, "What do I want to accomplish? How do I want my audience to be different after they have heard what I have to say?"
Instead of focusing on all the things you could say about your topic, focus on what your audience needs to know for you to get your point across. One helpful way to do this is to write down the questions they are likely to have about your topic and then jot down brief answers. You'll find that you've just generated most of the content you'll need for your speech.