Those words that strike chill into the heart of a speaker "Sorry, we're running a bit late - can you cut your speech in half?" - should cause you no concern (OK, maybe a little). In a well-run event, you should never have to face this challenge, but believe me, it will happen at some point, so you need to know how to handle it. The first thing is not to panic. Everything will be fine, and your audience will still love you at the end of your speech. Hopefully, you will get the warning in sufficient time to make adjustments.
Here's what you shouldn't do - finish your speech early. There is no point in getting halfway through your prepared presentation and then saying "That's all I have time for. That really would be short-changing your audience. Instead, you need to take a good look at your speech and focus on the essential points. Keep the opening and closing statements, and at least one good story. Everything else is optional.
If you have audience exercises, dump them, unless they are a just a quick vote. Give even more emphasis to your core message. You don't need to apologise for rushing through your speech, since you will go at your planned speed through less material. If you have slides, consider not using them. You can easily talk for ten to fifteen minutes without them. Shorten or drop any Q&A session, and make yourself available during the next break.
Treat the "emergency" as an opportunity. If you do it well, the organiser will be extremely grateful to you, and so will your audience. It's a chance to demonstrate that you're a real professional.