Phrase for doing something unexpectedly fast [closed]

Overnight is a perfectly idiomatic term for something that is completed much faster than expected (and, of course, something that would be expected to take much longer than one day).

For example: "Popular support for the war crumbled overnight, once the government's deception was revealed".

If you want an idiomatic term, you could use lickety-split

With great speed.

American Heritage

In a flash.

Also, in a jiffy or second or trice . Quickly, immediately. For example, I'll be with you in a flash, or He said he'd be done in a jiffy, or I'll be off the phone in a second, or I felt a drop or two, and in a trice there was a downpour. The first idiom alludes to a flash of lightning and dates from about 1800. The word jiffy, meaning "a short time," is of uncertain origin and dates from the late 1700s (as does the idiom using it); a second, literally one-sixtieth of a minute, has been used vaguely to mean "a very short time" since the early 1800s; and trice originally meant "a single pull at something" and has been used figuratively since the 1500s.

in a flash