You can speak about three words a second. Which means if you are asked to present for 5 minutes then you need to deliver 900 words.
One of the best ways to improve as a speaker is to script what you are going to say (even if you end up not delivering the script when you talk). The process of writing out your thoughts will help you formulate a better flow through your presentation.
Once I have my slide deck ready, I'll rehearse slide by slide. I record myself on my phone. I then transcribe what I said in the notes section of PowerPoint. I am always surprised by how repetitive I am on the first time around.
I then re-edit the notes: I try to reduce the number of words while maintaining the point I am trying to make.
Once the individual notes are ready, I then read through all the text as a single body. I check the word count and ask myself: can these words be delivered in the time allocated to me.
Wordcount / 180 = Number of the minutes these words will take to deliver.
If you are over your word count, you will need to edit, and this can be hard; often you will have to throw away your favourite sections, or your best jokes.
Editing is important. Being a great speaker isn't just about what you say, it what you choose not to say.