To expand on this... Spoken English should be treated as a separate dialect from Written English. They don't entirely share the same grammar, they have very different pools of idioms, and their concepts of effective use are almost entirely disjoint. Be aware that this difference exists, and practice your speeches out loud. Good essays generally do not make good presentations, and good presentations are almost universally terrible essays.
In particular, while Written English favors being concise, Spoken English favors explicitness, often to the point of redundancy. While a reader can go back and re-read a complex passage, and unpack it at leisure, during a speech there is no option but to parse it in realtime. Simple sentence structure, explicit large-scale structure markers, and outright redundancy are all markers of good Spoken English that your Written English instructors in school probably beat out of you.
Perhaps that's true, but I wonder about the great timeless speeches that also hold there own as essays. The Gettysburg Address, Churchill's speeches, JFK's inaugural address, Colbert or Jobs or D.F. Wallace's commencement speeches, etc.
Were they so good that they worked in both forms?
Or maybe they weren't good as speeches. I wasn't there for any of them.
Keep in mind that GP's advice isn't aimed towards well-practiced speechwriters and eloquent speechmakers. It's to people who are nervous even about joining Toastmasters.