Flat-out and Very fast

Flat-out adjective - Having no exceptions or restrictions.

Very fast and flat-out are semantically related. In some cases you can use "Very fast" instead an adverb phrase "Flat-out".

Very fast adverb - In a relatively short time.

Flat-out and very fast are semantically related. Sometimes you can use "Flat-out" instead an adverb phrase "Very fast".