Need to write a short speech? Don't write it.
I'm not being sarcastic. Should you research a short speech? Absolutely. I'm not saying, "Go out there and wing it."
You should prepare. The old line that every minute of a speech requires an hour of research and practice still applies. Preparation and practice are essential.
The pitfall I'm talking about is writing the full text of a 3-to-5 minute speech.
It's a mistake, because the person giving that speech will stand up there and try to deliver it word-for-word, and unless they're a seasoned speaker who can glance at full text and treat it as notes, it'll be a disaster.
Short speeches are different animals. A keynote speech requires full text and maybe a teleprompter, because nobody should wing what might be the most important 30 minutes of their life.
A short speech will come off as wooden if you try to read them word-for-word.
I've seen audiences get bored in the first ten seconds of a three-minute speech once the speaker started reading text. If you lose them in the beginning, you usually don't get them back.
So how can you write something that shouldn't be written word-for-word?
Outline it. Practice it. Refine the outline and practice again.