Delivering speeches is not about showing off your extensive vocabulary. There's no need to try to impress audiences with the length of words that you know. Consider the following paragraph from Richard Dowis, author of "The Lost Art of the Great Speech";
"Short words can make us feel good. They can run and jump and dance and soar high in the clouds. They can kill the chill of a cold night and help keep us cool on a hot day. They fill our hearts with joy, but can bring tears to our eyes as well. Small words of love can move us, charm us, lull us to sleep. Short words give us light and hope and peace and love and health - and a lot more good things. A small word can be as sweet as the taste of a ripe pear, or tart like plum jam."
Each word in that paragraph is just one syllable, yet they still have the power to evoke powerful feelings and strong emotions. Speechwriters know that short, simple words are often the most powerful elements of great speeches, and brilliant speakers know that too. In the words of Ralph Waldo Emerson : "An orator is never successful until they learn to make their words smaller than their ideas"
When you prepare a speech, look at the words you are using. Could they be simpler? Is there a way of expressing your ideas in shorter words? You may well find that the impact is more powerful when you trim it down.