With the updated software, I was really cooking with gas. I got the project done in half the time.
Usage notes
Grammatically, a use of the "-ing" form of cook + the prepositional phrase "with gas". Together with a form of be this forms a progressive verb predicate. Only rarely are other forms used in this idiomatic sense.
It is also possible to substitute "gas" with another cooking or combustible material in order to create an exaggerated or contrary meaning of the phrase. For example, using "cooking with newspapers" would indicate that the concept referred to is incredibly inefficient, while "cooking with enriched uranium" would indicate the opposite.[1]
From the suggestion, heavily advertised in the late 19th and early 20th centuries, that stoves using natural gas as a fuel cook more effectively than, for instance, wood-burning or electric stoves.