What a boom!

How are you?

I've been listening to a radio show to study English every morning,
and I wrote down what an American teacher said during the show today.
What he says might sound somewhat childish, but that's just the for sake of
our dictation practice.
Anyway, it is as follows.

" You know, not many people know this,
but I have a special skill I’ve developed over the years.
I’m a speed reader.
I can read the entire English language dictionary in twelve seconds.
What a boom.(?) Very fast."

Not that hard to understand but the last one - what a boom.
I've looked up the dictionary to see the definition of 'boom'
and there were many.

1. the resonant cry of animals
2. a rise in popularity
3. a period of rapid economic growth
4. a rapid increase in price, numbers etc.

Though the forth definition might be, I guess, the best of all,
I can't surely tell which one.

'What a boom!'

Is it more like 'How fast!' in the context?

I'd appreciate any of your comments.
Thank you.