It is common sense, really; the faster you speak, the harder it is for your listener to understand. The easiest way to instantly increase the clarity of your speech is to simply slow down.
As you speed up, you make compromises in your articulation. You cut off the ends of words and shove words together. Sentences also run together, making complete thoughts hard for a listener to decipher.
When speaking in public, audiences generally prefer a speaking rate of around 200 words per minute. Casual conversations and meetings with individuals one-on-one are generally much faster. A rule of thumb: the more formal the presentation (and the larger the audience), the slower your speaking rate should be.
A client of mine once said after listening to someone speaking at the proper rate, “But it would just be PAINFUL for me to speak that slowly.” “Good,” I said, “It should be painful at first. Otherwise you’re still speaking too fast!”
That said, be careful your rate doesn’t end up being too slow. You could sound tired, bored or as if you’re speaking in a condescending manner to your audience.