Now I know you could ask someone to say some word or sentence fast.
The white man asked him if he knew just where he was, and said, "Well, down here, you say 'mister' and you say it snappy, you hear." The colored man replied, "Now, I don't say 'mister' to nobody."
Source: Southern Humor, Duke University Press, 1995
But what if someone, let's say your son, wants to say how something has happened. You are in rush or you don't feel like listening to a long story. In a very informal speech, can you we say:
Okay, but say it snappy.
You better say it snappy.
Cut to the chase is not an option because I would say it when I interrupt someone who otherwise would go on about something too much.
Also, what other idiomatic expressions you might say in a scenario like this?